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Madge Training TRN-02A

Self-Study Guide COPYRIGHT ©1997 Madge Networks Ltd. All rights reserved. No part of this publication may be reproduced, transmitted, transcribed, stored in a retrieval system, or translated into any language or computer language, in any form or by any means, electronic, mechanical, photocopying, manual or otherwise, in whole or in part, without the prior written consent of Madge Networks, Inc.

NOTICE The information contained in this document is subject to change without notice. Madge Networks, Inc., reserves the right to revise this publication and to make changes from time to time in the content hereof without notice.

DISCLAIMER While every precaution has been taken in the preparation of this document, Madge Networks Ltd., assumes no responsibility for any errors or omissions that may appear in this document. Nor does it assume any liability for any damages resulting from the use of the information contained herein.

TRADEMARKS ©1997 Madge Networks, Ltd. All rights reserved. Madge and the Madge logo are trademarks, and in some jurisdictions, may be registered trademarks of Madge Networks. All other brand and product names are trademarks or registered trademarks of their respective holders. Madge Training TRN-03 - Multisegment Token Ring ______

Table of Contents

1. COURSE PREREQUISITES AND OBJECTIVES ...... 3

1.2 COURSE OVERVIEW ...... 3 1.3 COURSE PREREQUISITES ...... 3 1.4 TOPICS YOU WILL COVER ...... 3 1.5 COURSE OBJECTIVES...... 4 2. COURSE MAP ...... 5

3. HOW TO USE THIS GUIDE...... 6

3.1 WHAT IS COVERED IN THIS GUIDE ...... 6 3.2 HOW TO USE THIS GUIDE ...... 6 4. REVIEW TOKEN RING BASICS ...... 6

4.1 ANSWERS TO REVIEW...... 8 5. BRIDGING FUNDAMENTALS...... 11

5.2 SERIAL BRIDGES ...... 17 5.3 PARALLEL BRIDGES ...... 18 5.4 REMOTE BRIDGES ...... 18 5.5 BACKBONE TOPOLOGY ...... 19 5.6 DUAL BACKBONES...... 21 5.7 BRIDGING METHODS ...... 22 5.8 TRANSPARENT BRIDGE OPERATION ...... 23 5.9 SOURCE ROUTE BRIDGE OPERATION...... 28 5.10 SOURCE ROUTE TRANSPARENT BRIDGES...... 30 5.11 WHAT MAKES A ROUTER DIFFERENT FROM A BRIDGE?...... 30 5.12 SELF STUDY: COMBINING BRIDGING AND ROUTING ...... 33 5.13 TEST YOUR UNDERSTANDING: BRIDGING FUNDAMENTALS...... 34 6. THEORY OF SOURCE ROUTE BRIDGING...... 36

6.1 SELF STUDY : - RING NUMBERS AND BRIDGE NUMBERS...... 37 6.2 RING NUMBERS AND BRIDGE NUMBERS - 3 RULES TO REMEMBER ...... 38 6.3 SELF STUDY - DRAW A FRAME WITH THE MAXIMUM NUMBER OF HOPS...... 40 6.4 TEST YOUR UNDERSTANDING: THEORY OF SOURCE ROUTING...... 43 7. THEORY OF SOURCE ROUTE BRIDGING: EXPLORER FRAMES ...... 44

7.1 SELF STUDY: ALL ROUTES EXPLORERS ...... 45 7.2 WHY USE ALL ROUTES EXPLORERS? ...... 46 7.3 DISADVANTAGES OF ALL ROUTES EXPLORERS...... 46 7.4 WHY USE SPANNING TREE EXPLORERS?...... 48 7.5 EXPLORATION STRATEGIES ...... 50 7.6 PROS AND CONS OF ARE OUT SR RETURN...... 51 7.7 PROS AND CONS OF STE OUT ARE BACK ...... 52 7.8 PROS AND CONS OF STE OUT SR BACK ...... 52 7.9 CONTROL HEADER FIELDS...... 52 7.10 BRIDGE DECISION PROCESS ...... 55 7.11 REVIEW OF EXPLORATION STRATEGIES: A REAL WORLD ISSUE...... 55 7.12 TEST YOUR UNDERSTANDING: EXPLORER FRAMES...... 57 8. SPANNING TREE...... 57

8.1 ADDRESSING FORMATS FOR SPANNING TREE FRAMES ...... 60 8.2 CANONICAL AND NON-CANONICAL ADDRESSING FORMATS ...... 60 8.3 ELECTION OF ROOT BRIDGE - THE BRIDGE ID...... 62 8.4 DETERMINING THE DESIGNATED BRIDGES - ROOT PATH COST ...... 65

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8.5 SELF-STUDY: PREDICTING THE SPANNING TREE...... 66 8.6 REVIEW SPANNING TREE ...... 67 8.7 TEST YOUR KNOWLEDGE: SPANNING TREE...... 68 9. RING AND BRIDGE NUMBER CONFIGURATION WORKSHOP ...... 69

9.1 PRE-REQUISITES FOR THIS WORKSHOP ...... 69 9.2 PRACTICAL OBJECTIVE...... 69 9.3 POINTS TO REMEMBER ...... 70 9.4 INSTRUCTIONS TO FOLLOW ...... 70 9.5 YOU WILL HAVE COMPLETED THIS EXERCISE WHEN...... 70 10. SPANNING TREE WORKSHOP ...... 72

10.1 PRE-REQUISITES FOR THIS WORKSHOP ...... 72 10.2 PRACTICAL OBJECTIVE...... 72 10.3 POINTS TO REMEMBER ...... 72 10.4 INSTRUCTIONS TO FOLLOW ...... 73 10.5 YOU WILL HAVE COMPLETED THIS EXERCISE WHEN...... 74 10.6 SPANNING TREE WORKSHOP: ANSWERS ...... 75 11. EXPLORER FRAMES WORKSHOP...... 76

11.1 PRE-REQUISITES FOR THIS WORKSHOP ...... 76 11.2 PRACTICAL OBJECTIVE...... 76 11.3 POINTS TO REMEMBER ...... 76 11.4 INSTRUCTIONS TO FOLLOW ...... 77 11.5 ANSWERS TO EXPLORER WORKSHOP ...... 80 11.6 EXPLORER WORKSHOP CONCLUSIONS...... 81 12. APPENDIX- TEST YOUR UNDERSTANDING: ANSWERS...... 83

12.1 BRIDGING FUNDAMENTALS ...... 83 12.2 SOURCE ROUTING THEORY ...... 85 12.3 EXPLORER FRAMES...... 85 12.4 SPANNING TREE...... 86 13. NOVELL® SOURCE ROUTE SERVER END STATION SOFTWARE: UPDATE ...86

14. EXPLORER FRAMELOGS FOR WINDOWS95 ...... 87

15. APPENDIX- GLOSSARY OF TERMS ...... 89

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1. Course prerequisites and objectives

1.1.1 Course code TRN-03

TRN-03 Multi-segment Token Ring 6 hours

1.1.2 Teaching method

Self-paced learning

1.2 Course overview

TRN-03 provides an introduction to principles of building multi-segment Token Ring networks followed by a detailed analysis of source route bridging.

We look at the possibilities of optimising the strategy of clients when setting up a connection to a server by analysing the use of broadcast frames.

The control of broadcast frames is a key strength of the Madge Ringswitch, hence a clear understanding of these principles is indispensable prior to learning how to configure the Ringswitch

1.3 Course prerequisites

Students are expected to have completed the following or equivalent courses:

TRN-01 Single segment Token Ring TRN-02 Madge hubs

1.4 Topics you will cover

• Key bridging techniques • Madge Smart Ringbridge • Transparent bridging • Bridge Configuration • Source Route bridging • Spanning Tree • The Routing Information Field • Exploration strategies • Hop count control • Multi-segment Token Ring design essentials

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1.5 Course objectives

By the end of this course you will be able to:

• Identify poor multisegment network design • List benefits of key Token Ring bridge topologies − Source Route bridging − Transparent bridging − Routing • Describe how Transparent bridges filter and forward frames • Describe how Source Route bridges filter and forward frames • State the 2 purposes of Spanning Tree for Token Ring networks • Draw and notate a simple correctly resolved Spanning Tree • Analyse Token Ring traffic flow using RIFs to determine network paths • Explain how use of RIF can provide load balancing over duplicate routes from client to server/mainframe • Use captured traffic to determine client exploration strategy for server connections − AREs - All Routes Explorers − STEs - Spanning Tree Explorers • Explain benefits of AREs, STEs and SR (specifically Routed) frames • Explain use of AREs and STEs by different providers • Assess broadcast strategies used in different client/server environments and implement measures to reduce ARE traffic • Configure ring numbers and bridge numbers on the Smart Ringbridge • Fault find basic spanning tree configurations using Trueview and the LCD display

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2. Course Map

Here is an outline of the learning activities for this course which can be taken in one day. This includes approximate timings. Use this information to pace yourself through the training. Course Map

9.00 Review of Token Ring Basics Basic Concepts of Bridging and Routing 10.30 Using Source Routing for establishing links between Token Ring stations

12.30 Lunch 1.30 Bridge Configuration Workshop 2.00 Spanning Tree Workshop 3.00 Explorer Frames Workshop 4.30 Review Workshops 5.00 Finish

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3. How to use this guide

3.1 What is covered in this guide

The Multi-segment Token Ring Module is a self-paced learning package aimed at engineers who understand Token Ring basics and wish to learn about Madge Token Ring switching. To do this you need to grasp the principles of source route bridging and transparent bridging with Token Ring.

ICON KEY

) Key 3.2 How to use this guide Information This manual includes a variety of activities which allow you to cover the topics in a Workbook way which will suit your own style of learning. Review " Self-Study Look out for the icons on the left. Try to cover all the suggested practical activities Lab Exercise and check your knowledge using the workbook reviews.

If you are taking Madge Certification tests make sure you cover the topics and objectives listed in the first chapter as well as the more detailed objectives for each module or chapter.

4. Review Token Ring basics

______Student Notes

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Before we start work on Bridging Concepts make sure you have grasped the basic concepts of Token Ring itself. This is covered in the first Token Ring course known as TRN-01. ) Only one of the possible answers is correct. Read all the answers carefully 1. Under token ring a “special” node is designated to be in charge of the ring and generating and controlling the token. This node is called the: a) Ring Error Monitor b) Active Monitor c) Configuration Monitor d) Active Report Server

2. The IEEE Token Ring standard occupies which two layers in the ISO model?

a) Layer 1 & 2 b) Layer 1 & 3 c) Layer 2 & 3 d) Only resides in layer 2

3. When a data frame is sent around a Token Ring:

a) It is copied into every station on the ring and then passed on b) It is copied into every active station on the ring and then passed on c) It is copied into the destination and then passed on d) It is removed by the destination and an acknowledgement is passed on

The functional address bits are used to allow: a) a node to adopt a single specific function e.g. Active Monitor b) a node to adopt multiple functions simultaneously e.g. bridge and network management (REM) c) network management applications to monitor and gather statistics

5. What is the principle function of MAC frames ?

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a) to maintain the operation of the ring b) fault diagnosis c) to carry data d) to purge the ring

6. Look at the diagram of a Token Ring frame below. The CRC check is done to ensure that

a) Any part of a frame received by any station has not been corrupted

b) The data portion carried by the frame has not been corrupted

c) The frame does not have the CRC bit set

d) Frame type, address and data in the frame have not been corrupted

CRC checked

1 byte 1 byte 1 byte 6 bytes 6 bytes4 bytes 1 byte 1 byte

Access Source FCS SDEL FC Destination DATA EDEL FS Control Address Address (CRC)

Frame type: Last Bit (EDI) - MAC frame Error Detected - Data frame Optional RIF Priority Frame Check Sequence Token or Frame ? Active monitor ? Address Recognised ? Frame Copied ?

4.1 Answers to review

1. Under token ring a “special” node is designated to be in charge of the ring and generating and controlling the token. This node is called the:

______Student Notes

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a) Ring Error Monitor b) Active Monitor c) Configuration Monitor d) Active Report Server

The first active adapter or node on the ring will become the Active Monitor and will manage tokens and frames and generally maintain the ring.

2. The IEEE Token Ring standard occupies which two layers in the ISO model?

a) Layer 1 & 2 b) Layer 1 & 3 c) Layer 2 & 3 d) Only resides in layer 2

The Token Ring standard specifies cabling types and signalling at layer 1-the physical layer - and a token ring passing protocol for media access control (MAC) at layer 2 - the datalink layer. A high sublayer (LLC) is provided as 802.2 which provides access to layer 3 protocols like IP and IPX. This is not strictly part of the Token Ring protocol, but in practice MAC and LLC are inseparable.

3. When a data frame is sent around a Token Ring:

4. The following diagram describes certain so-called functional addresses available on certain token ring stations Functional Address Bits Active Monitor (C000 0000 0001) Ring Parameter Server (C000 0000 0002) Ring Error Monitor (C000 0000 0008) Configuration Report Server (C000 0000 0010) NetBIOS (C000 0000 0080) Bridge (C000 0000 0100) The functional address bits are used to allow: a) any node to adopt a single specific function e.g. Active Monitor b) a specific node to adopt a single specific function e.g. Active Monitor c) certain nodes to adopt functional roles e.g. bridge or network management d) network managers only to monitor and gather statistics

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5. What is the principle function of MAC frames ? a) to maintain the operation of the ring b) fault diagnosis and recovery c) to carry data d) to purge the ring

6. Look at the diagram of a Token Ring frame below. The CRC is check done to ensure that.

a) Any part of a frame received by any station has not been corrupted

b) The data portion carried by the frame has not been corrupted

c) The frame does not have the CRC bit set

d) Frame type, address and data in the frame have not been corrupted

______Student Notes

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5. Bridging Fundamentals

By the end of the session you will be able to...

State the purpose of a bridge

List bridge topologies and types – eg serial, loop, backbone, remote List the merits of the backbone topology

Compare and contrast bridging methods – source route – transparent Explain the difference between a bridge and a router

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What is the purpose of a Bridge?

1 Ring A Ring B 6

Connects two physical rings Forwards or Filters Frames Single logical network Keeps local traffic local

Figure 1. Purpose of a bridge

5.1.1 What is the purpose of a bridge?

The purpose of a bridge is to connect two (or possibly more) rings into one logical network. This means that when station 2 sends frames to station 3, stations on ring B will not see the frames.

In other words local traffic does not cross the bridge, it remains local.

However when station 1 needs to talk to station 6, the bridge will allow this. So a bridge will forward or filter a frame as required. In general the end stations are unaware that the bridge is performing this task on their behalf.

There are other specific benefits to be gained from bridging between Token Rings. " Use your knowledge of Token Ring to list these benefits. Use the hint words in the table to get you thinking.

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• Flexibility

• Performance

• Reliability

• Compatibility

• Security

• Overcoming cabling distances

Turn the page when you are ready to review your answers.

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Benefits of bridging with Token Ring

Flexibility Comaptibility – Different network – More rings so increased speeds/types number of nodes – 16Mbps to 4Mbps – Token Ring to FDDI – 255 on STP, 144 on UTP Security ring – Filters by address or Performance protocol – Increase bandwidth Overcome cabling distances – Each ring has its own token – Remote Link greater than Reliability 2KM – MAC Processes are per ring – If one ring beacons, no effect on the other

5.1.2 Benefits of bridging with Token Ring

There are a great many benefits to be gained from linking Token Ring segments using bridges.

Segmenting a ring with a bridge provides you with 2 separate rings. This means you can increase the number of nodes on the network, with up to 255 nodes on an STP ring and 144 on a UTP ring.

The MAC processes run independently on each Token Ring and each ring has its own Token. This means two things: firstly performance is improved as each station gets increased access to the token, thus increasing that station’s share of the 16 Mbps bandwidth.

Secondly, any failure, whether it be soft errors or beaconing, is confined to the ring where the problem originated. This means, for example, that a troubleshooting ring could be created where potentially faulty devices could be isolated without affecting connected rings. This means the reliability of all rings can be maintained.

Bridges generally work most effectively when connecting LAN segments of the same speed and network type. A bridge can be used to link a 16Mbps ring with a 4Mbps ______Student Notes

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The down side of this is that in some cases frames will have to be truncated to 4K from a larger frame size. This means additional processing as the frame must be stored and then divided up with new token ring headers and trailers being added. Security is a key feature that a bridge is able to provide. The bridge software can analyse the MAC and reject a frame based on its MAC address, thus blocking traffic from a specific workstation. It can also look within the LLC header which contains the SAP (Service Access Point). This indicates whether a frame is destined for an IPX, IP or other stack. This allows the bridge to confine certain protocols to one side of the bridge.

Remote links can be used between sites to overcome the cabling distance limitss of Token Ring. CAUs for example cannot be further than 2Kms apart and require fibre connections to achieve this.

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Bridge Topologies

Serial bridges

Parallel bridges

Rem ote bridges

Much of network design is down to common sense and experience.

Apply your existing knowledge and assess serial, parallel and remote topologies according to the criteria in the leftmost column: " Serial Parallel Remote

Reliability and Availability Performance (Hop Counts, Filtering, Placing of Servers Maintainability (Effect of adding new rings)

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Bridge Topologies

Serial bridges Frames must cross rings Hop count limitations If one bridge breaks, lose communication

Parallel bridges

Alternative routes Load balancing (in Source Route bridging) Redundancy

Remote bridges Overcome physical distance limitations Counted as one hop

5.2 Serial bridges

Small token ring networks often grow by adding new rings in series with existing rings. This is a simple installation task and involves only the cost of a bridge and two cables. But there are several disadvantages: a frame may need to make several hops to reach its destination, each bridge adding delay and thus decreasing the performance of the application; any bridge or ring in the path between the workstation and the server can be a single point of failure; if you continue to install bridges in series, frames will soon encounter the hop count limit (7 hops with IBM bridges) - this means that a workstation cannot communicate with a server if the frames need to cross more than 7 bridges to get there, as the frame will be discarded after 7 hops.

Incidentally, you will hear the term latency used to describe the delay introduced by a bridge as a frame crosses it. A traditional token ring bridge is a Store and Forward device. The bridge checks a flag in the MAC header. If the frame is to be bridged the frame is then stored in memory; the integrity of the frame is checked by recalculating the CRC for the frame and comparing it with the existing CRC - if there is no match the frame is discarded; the frame is forwarded when the token becomes available on the destination ring. . ______Student Notes

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Clearly all of these actions will add delay to each frame as it crosses each bridge. Additionally since each frame., large or small, must be stored and checked the overall latency will vary slightly dependent on frame size.

Each additional bridge hop will add to additional latency and diminish the performance of applications running between clients and servers which use that path to maintain sessions.

5.3 Parallel Bridges

A solution using parallel bridges is more expensive than one using bridges in series, simply due to the amount of hardware required. Configuration is also more complex.

However, this is outweighed by the benefits of redundancy and load balancing, in which frames have alternative routes to their destination.

Redundancy means that a standby bridge is waiting to take over in case the active bridge fails. This works with both transparent and source route bridges.

Load balancing means that traffic between two rings can be shared by both bridges. This decreases the burden on each bridge and allows each workstation to find the fastest path to its destination. Load balancing works only with source route bridges.

5.4 Remote bridges

Remote bridges are deployed to overcome large physical distances between rings. In the case of Madge CAUs 2 kilometres is the maximum distances between CAUs on a ring using fibre cables.

Remote bridges operate in pairs or “bridge halves”. Each bridge half has two physical interfaces. A token ring interface allows the device to insert into the local ring. It has a device driver which operates normal bridging software, source route or transparent.

The bridge also has a remote interface which passes the frames to the second bridge half. This link could be any type of remote link such as ISDN, Megastream or T1 and is simply a pipe for the data frames to be bridged.

Between the two interfaces the bridge provided software to translate between the different frame types.

When a frame crosses a remote bridge this counts a single bridge hop.

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Backbone Topology

Ease of growth Reduced number of hops Centralised positioning of servers Well suited to large networks

5.5 Backbone Topology " First of all, take some time to analyse the benefits of the backbone network. Reliability and Availability Performance (Hop Counts, Filtering, Placing of Servers) Maintainability (Effect of adding new rings)

Turn the page to see some of the possible responses to this exercise.

Reliability and • Users can easily be placed on another ring in case of Availability failure of local ring.

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• One central ring is used as a common path for all traffic between rings. Backbone is single point of failure, but a dual backbone overcomes this problem. Performance (Hop • The number of hops is small and predictable: 1 hop Counts, Filtering, onto the backbone (where servers may be located) or Placing of Servers) 2 hops onto any other ring (also where servers or gateways may be located).

• It is possible to add further rings to the sub rings, but this creates hierarchies; the further from the backbone you are the more hops your frames have to make to reach any services, so performance will be diminished.

• A backbone is well suited to large networks where servers can be placed on the backbone or on specific sub-rings. In either case the total latency (due to bridge hops) can be easily predicted.

Maintainability • Easy to grow - each bridge allows one new ring to be (Effect of adding new added. rings)

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5.6 Dual backbones

A 001 B

A 002 B BB1 BB2

A 003 B

A 004 B

Dual backbones are probably the most popular types of network to be found on large Token Ring sites. This is because of the extra resilience provided by the second backbone, which also provides an extra path between all rings.

An example of this would a user on 001 wishing to reach a server on 004. Let’s assume the normal path might be via bridge A and BB1. If ring BB1 or bridge A becomes unavailable the path via bridge B and BB2 can be used instead.

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Bridging Method

OSI Layers 7 Application 6 Presentation 5 Session 4 Transport 3 Network Routers

Transparent Bridges 2 Data Link Source Route Bridges Source Route Transparent Bridges 1 Physical Repeaters

5.7 Bridging Methods

To clearly understand the role of bridges we will first compare bridges with repeaters and routers.

Repeaters function at the physical level (level 1) and simply regenerate the electrical signals at bit level. This is a means of extending the physical network and does not segment the network in any way. Whether applied to Token Ring or Ethernet there are specified limits to the number of repeaters that can be deployed.

Bridges use information in the MAC (or LLC) headers to determine whether to forward a frame. This means they work in a way which is hidden from network layer protocols like IP and IPX; they need have no awareness that this is how traffic is being passed round the network. Bridges, whether transparent, source route or hybrid occupy the Data Link Layer, layer 2, of the OSI model.

Routers are devices which can only handle frames by examining the network header. A node does not need the services of a router if the target node is on its own subnet. If the node cannot match the target subnet address with its own it will send a routing request to a router on its subnet. We will discuss routing in a little more detail later. Transparent Bridge Operation

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Addr Port Addr Port A 1 A 1 B 1 B 1 B C 1 C 1 E D 2 D 1 E 2 E 2 F 2 F 2

A Ring A Ring B Ring C 121 2 1 2

C D F

Each bridge has a table containing – Destination addresses it knows – Port on which that address can be found

5.8 Transparent bridge operation

A transparent bridge maintains a table which stores each known address with the port on which the node with that address can be located. The process of forwarding the frame is thus very simple.

______Student Notes

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Transparent Bridge Operation

HDRHDRDADASASALLCLLCDATADATATrailer

Record SA in Address Table

Is DA in Address NO: Forward frame to all other ports. Table?

YES: Are SA and DA NO: Forward frame on known port on same port?

YES: Discard frame

Once the bridge enters forwarding mode it has to build a picture of all local and remote nodes and put the information in the Address Table. The flow chart above describes this. Once a frame has arrived at a bridge port the bridge records the address of the frame in the Address Table. Let’s use the diagram on the previous page as an example. When stations A, B and C insert into ring A, Ring Poll frames will be generated which will be seen by the bridge. The bridge thus discovers that these stations are to be found on port 1 and builds the bridge table accordingly.

That allows us to understand how the addresses of stations on directly attached rings are discovered by the bridge. What about other stations?

In fact, the bridge is not concerned whether frames arrive from directly attached stations or remote stations. Addresses are learned by inspecting the source addresses of all frames on the segment, even if these frames were not generated by stations directly attached to the local ring. Consider the situation where bridge 1 has started up and acquired the addresses of the directly attached stations on ring A due to Ring Poll. The next event to occur will be that requests are sent by the stations to contact servers, which may be several hops away.

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When station A sends a request to talk to server F (for example) bridge 1 will forward the request on all ports (port 2 in this case). This is because F’s address is not in the table. Bridge 2 will receive the request frame and likewise it will forward it on all ports. Server F will then start returning frames to station A via bridge 1. Bridge 1 will then record the source address in its Address Table, with port 2 as the port on which the frames were seen.

Once the session between A and F has been established, bridge 1 will have discovered that station F can be reached on port 2 and this will be placed in the table.

5.8.1 Source and destination address on same port?

The final part of the transparent bridging logic is a check to see if the destination address for the frame is on the same bridge port as the source address. If this is the case then the bridge must not forward the frame as the frame originated from that segment.

5.8.2 Transparent bridging support with the Madge Ringswitch

You may like to apply this information to a real life switch or multi-port bridge. The Madge Smart Ringswitch is not part of this training module but it is interesting to note that this device can store up to 10,000 MAC addresses in its table.

This number is so large simply because the switch must keep track of all active stations, including those not directly attached to the switch. The switch must know through which port it can forward each packet to ensure delivery.

" 5.8.3 Self-study exercise Look back at the last two slides and examine the bridge tables for bridges 1 and 2. Account for each entry in each table. For the purpose of illustrating your point you can treat any of the nodes as workstations or servers.

Hint: “A” could be a Netware server.

1. Why do addresses A,B,C and D appear in the table for bridge 1?

______

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2. What caused addresses E and F to appear in bridge 2’s table?

______

3. Describe how address A might have come to appear in bridge 2’s table.

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Answers 1. Why do addresses A,B,C and D appear in the table for bridge 1?

Addresses A,B and C appear as available on port 1 and D appears as available on port 2. The addresses have been learned by the bridge when it receives its first frames from these nodes. In the case of token ring, Ring Poll frames (AMPs and SMPs) would have told the bridge that these stations are local on port 1.

2. What caused addresses E and F to appear in bridge 2’s table?

Same as 1. These are stations local to the bridge on port 2.

3. Describe how address A might have come to appear in bridge 2’s table.

“A” could be the address of a server which station E on ring 3 is logged into. Under IPX a SAP request is sent out by station E with “A” as the preferred server. Initially address “A” is not present in bridge 2’s table. Bridge 2 forwards the frame. Bridge 1 receives it and forwards it on the ring where server “A” is inserted .

Server A responds and the response frames appear on port 1 of bridge 2. Bridge 2 will keep on seeing frames from A on port 1 and the entry in bridge 2’s table will be kept until traffic from A ceases to appear, or shortly thereafter.

5.8.4 Address ageing

In fact the entry will not disappear immediately from the table. A so-called “ageing timer” of between 20 and 30 seconds applies after which the address is finally removed. During this period a new frame from the same source can arrive which will reset the ageing timer to 0.

______Student Notes

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Source Route Bridge Operation

RIF

B HDR DA SA Ring/bridge pairs LLC DATA Trailer E

A Ring A Ring B Ring C 121 2 1 2 Station

C D Server F RIF built up as explorer frame is broadcast across the rings

Frame reaches server with complete RIF

Server uses RIF to get back to station

5.9 Source Route Bridge Operation

As we mentioned earlier, routing and bridging are different, the key distinction being that routers use information in the level 3 network header, and bridges use information in the MAC or LLC.

So why are we suddenly talking about Routing again? Source Route Bridging is not similar to Routing in any sense. Source Route Bridging uses a number of techniques initiated by the workstation which wishes to start a session with a server. The initiating station is the “source” of this route determination activity. “Routing” refers to the technique of recording the route taken by the frame as it passes over each bridge, and using this information later for finding a path.

This means that unlike transparent bridges where information is held in the bridge tables, a source routing bridge stores information in the Data frame. Initially, a station wishing to perform source routing sets a flag in the Token Ring header to indicate that the frame must be processed by Source Route bridges. This flag actually indicates the presence of a RIF or Routing Information Field. We will discuss the precise location of this flag later. ______Student Notes

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If there is no RIF at this point the two stations wishing to start a session must establish a route and fill in a RIF. This process is called route discovery and will be covered in full later. Once fully formed, the RIF will contain a list of each ring and bridge the frame must cross to reach its destination, normally a server of some kind.

Let’s use the diagram above as an example of this. Imagine a frame passing from A on ring A to server F on ring C. What ring and bridge information needs to be stored in the Routing Information Field to let the frame pass? " RIF Indicator Ring Bridge Ring Bridge Ring Bridge set? No. No. No. No. No. No. 0

As you can see from the answers at the end of the section, the only possibly confusing part is that the final bridge number is actually 0. This is because a frame is always destined for an end station on a ring. Therefore, when traversing a ring a RIF must be terminated with a 0.

You should realise that you have almost certainly sat in front of a station which performs Source Routing to reach its server, you may even have loaded or configured the drivers to do this. For example, have you ever loaded the following drivers? Novell Madge IBM LSL SMART IPX SR=Y DEVICE=DXMA0MOD.SYS TOKEN DEVICE=DXMCMOD.SYS IPXODI DEVICE=DXMTMOD.SYS ROUTE NETX

In each case you have loaded source routing, in the last case it is built into the low level driver. This is because IBM workstations need to talk to IBM mainframes which expect to be installed on a source route bridged token ring network. Answers to quiz RIF Indicator Ring No. Bridge No. Ring No. Bridge No. Ring No. Bridge set? No. Yes 00A 1 00B 2 00C 0

Source Route Transparent Bridging

______Student Notes

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Frame arrives at bridge port

YES: bridge the frame using Source Routing

Marked for Source Routing?

NO: bridge the frame using Transparent

5.10 Source Route Transparent Bridges

Many bridges, including the Madge Smart Ringswitch, can perform either Transparent or Source Route Bridging. On a bridge such as this, if the port is enabled for both Transparent and Source Route bridging the frame follows the flow diagram above.

The bridge checks the RIF indicator flag. A frame with it set will be source routed, otherwise the frame is transparently bridged.

5.11 What makes a router different from a bridge?

Before we conclude our introduction to the principles of bridging with Token Ring let’s revisit our definitions of routing and bridging from earlier.

Bridges use information in the MAC or LLC headers to determine whether to forward a frame. This means they work in a way which is hidden from network layer protocols like IP and IPX.

The key differences between a bridge and a router can be summarised as follows: ______Student Notes

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Routers tend to connect networks of differing topologies e.g. Token Ring and Ethernet. They join networks with different network numbers or subnet addresses. Bridges tend to connect segments of the same media, and with the same subnet address.

Routers support specific network protocols (IPX, IP, AppleTalk) and allow internetworking to occur between subnets which support one of these layer 3 network types.

Routers do not perform a hidden role. A station must decide that a frame is not on his subnet, and then send a specific frame to routers on the subnet. A router only handles packets addressed to itself. Only when a frame has been sent to the router itself can the router forward the frame to the next hop. The final router will pass the frame to the specific network where the end station resides. All routers need some method of discovering the next hop to a particular subnet. An example of this is RIP, Routing Information Protocol which provides Router Updates on a routine basis and whenever the topology of the network changes.

Bridges are different from routers in all these points as they operate in a way which is hidden to all layer 3 protocols which are specifically supported by routers.

The following slide summarises these points on routers versus bridges: Routers versus Bridges

______Student Notes

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Handle frames based on Handle frames based on Network Header LLC/MAC Header

Tend to connect networks Tend to connect networks of differing LAN types (or of same LAN type LAN to WAN) Support any LLC based Support specific protocols: protocol - network layer is IP/IPX/Appletalk unaware

Protocol required for Protocol required to updating routing maintain Spanning Tree

information – “loop avoidance” protocol

______Student Notes

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Routing /Bridging Combination

Net Ware IPX Segment 000002 Client

Load ROUTE.COM

Source Routing Bri dge

IPX Segment Ri ng A Ri ng B 00000A

Net Ware Server/ Router

Load ROUTE. NLM

" 5.12 Self study: Combining Bridging and Routing In order to fully understand the concepts of bridging and routing in a Novell environment we can look closer at a simple but typical Novell network.

Look at the diagram and try answering the following questions:

1. How many IPX segments (networks or subnets) can you see on the diagram? ______

2. The router on ring B receives frames from segment 00000A. It needs to pass these frames to a station on ring A. What end station software is needed at both ends? ______

3. What does ROUTE.NLM do?

______

4. What is the difference here between the role played by the router and that played by the bridge? ______

5.12.1 Answers to Self Study

1. How many IPX segments (networks or subnets) can you see on the diagram? 2 segments, 000002 and 00000A routed by the Novell server ______Student Notes

Copy of Sg_trn02A  Madge Networks Page 33 of 93 Madge Training TRN-03 - Multisegment Token Ring ______

2. The router on ring B receives frames from segment 00000A. It needs to pass these frames to a station on ring A. What end station software is needed at both ends? ROUTE.NLM is needed on the server end and ROUTE.COM on the workstation end.

3. What does ROUTE.NLM do? ROUTE.NLM is the same as ROUTE.COM on the workstation i.e. it sets the RIF indicator flag so that the bridge knows to use a RIF. ROUTE.NLM has the job of exploring possible routes to reach ring A. These drivers have nothing do with routing they enable source routing in the frames which allows the bridge to pass the frame and fill in the RIF.

4. What is the difference here between the role played by the router and that played by the bridge? ______

Routing occurs at level 3 - IPX frames are routed between IPX segment 00000A and 000002.The same frames are then source route bridged in order to reach ring A. IPX segment 00000A could be any MAC type (Token Ring, FDDI, Ethernet).

5.13 Test your understanding: Bridging Fundamentals

Complete the following self-test by answering the following questions. Check your answers are by using the answer key located in Appendix A.

In the multiple choice tests only one of the possible answers is correct. Read all the ) answers carefully

1. What type of device is hidden from layer 3 protocols like IPX? a) A repeater b) A transparent bridge c) A source route bridge d) All of the above

2. A router works at which layer of the OSI model a) Physical b) Datalink c) Network d) Transport ______Student Notes

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3. Which of the following is a benefit of a traditional bridge between network segments? a) a cabling fault on one ring can never be passed to the second ring b) a bridge can easily link Token Ring and Ethernet c) bridges add an insignificant delay (latency) as frames pass through them d) none of the above

4. A device which copies frames based on the source and destination MAC address is called a) a repeater b) a router c) a transparent bridge d) a source routing bridge

5. Why is the following RIF invalid? Ring No. Bridge No. Ring No. Bridge No. Ring No. FFF 1 00B 3 0

6. A correctly formed single backbone topology provides a) alternative routes from client rings to the server b) single hop between any two rings c) single hop to the backbone (server) ring d) all of the above

7. In a Token Ring frame the RIF is a) present if requested by the bridge b) present if requested by end station software c) optional if the RIF flag is set d) mandatory

8 A transparent bridge acquires addresses and assigns each address to one of its ports. It does this by a) Examining the source and destination address b) Examining the source address only c) Examining the destination address only d) None of the above

______Student Notes

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6. Theory of Source Route Bridging

By the end of the session you will be able to...

Define valid Ring and Bridge numbers

Explain how the Routing Information Field (RIF) is formed

State how clients and servers use the RIF

Describe the benefits of each Route Discovery Method

Log Route Discovery session and compare theory with practice

During this session you will learn the basic rules that end-stations and bridges have to follow to perform source routing. You will look at the different Route Discovery Methods and see how they can provide benefits such as load balancing without having a severe negative impact on performance.

______Student Notes

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Theory of Source Route Bridging

1 2 0011 0023 00F 4 Parallel 1 1 Bridges

003 A A 101 Remote Bridge

6.1 Self Study : - Ring numbers and Bridge numbers

At this moment you are not expected to understand any of the details of how source routing works. However with the knowledge you have picked up, spend a few moments studying the above diagram.

Ask yourself if the network is valid and would actually function. " 1. Are the bridge numbers valid? 2. Are the ring numbers within specification? 3. Why are there 2 bridges called “A”?

If you found this difficult, do not worry, the topic is fully covered over the next 3 pages.

Turn the page for answers to these questions.

______Student Notes

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Answers to quiz 1. So that unambiguous paths are recorded in each RIF; bridge numbers between the same 2 rings must be unique. Otherwise bridge numbers need not be unique. 2. Ring numbers must always be unique 3. Bridge A is made of two bridge halves. In fact, from the point of view of the token ring interface on each bridge half, this is a single bridge. The medium for transmitting frames from one ring to the other happens to be a phone link (T1 or similar) rather than the internals of a local bridge.

6.2 Ring numbers and bridge numbers - 3 rules to remember

Source route bridging is achieved by close co-operation between end-stations and bridges.

Source routing has to be requested by the end-station. In the case of Novell this means loading ROUTE.COM, ROUTE.NLM or something similar. It is the bridges which do all the hard work after that. Since each bridge adds to the RIF of an explorer frame as it crosses the rings, it is the bridges which must be configured with the ring and bridges numbers.

Let’s list the basic rules of ring and bridge numbers.

1. Every ring must have a unique ring number consisting of 3 hex digits. Valid ring numbers range from 000 to FFF.

2. Every bridge must have a number consisting of 1 hex digit. Valid ring numbers range from 1 to F. 0 is invalid as this is used to mark the end of a RIF. Bridge numbers only need to be unique when a bridge is joining 2 rings in parallel with another bridge.

3. So that RIFs are always unambiguous the combination of Ring Number + Bridge Number + Ring Number must unique on the bridged network.

This means that the diagram on the previous page is indeed correct with its " duplicated bridge numbers. Don’t forget there are only 15 valid bridge numbers, so a larger network is bound to have duplicated bridge numbers.

Now we have laid down the basic rules we can examine in detail the process of route discovery and the roles played by the stations and the bridges. The slide below shows in some detail how a station indicates that it wants its frames source route bridged. Please don’t worry if you don’t understand it straight away, we are going to discuss this in detail shortly.

______Student Notes

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Flagging the frame for Source Routing

Token Ring HDR DA SA RIF LLC DATA Trailer Frame

Individual/Group bit in the IUManufacturer ID Serial # Source Address is GL always Individual ...

R U So use it to indicate I Manufacturer ID Serial # presence of a RIF F L

Example: 0000F6123456 No RIF 8000F6123456 RIF present

The real point behind all this complexity is that the station needs to locate an unused bit somewhere in the MAC frame to use as an indicator that the frame needs to be source routed. The source routing specification says that a certain bit should be used.

In fact we use the high order bit of the source address. This can in theory be used to indicate whether the address is a group or an individual address. We know that a source address is always an individual address, as a group address can only be used for destinations.

That’s why, if we analyse a source route bridged source address, we see 8000F6 etc (in the case of a Madge address). Fortunately, we don’t normally see this - the detail is handled by the network analysis software, in our case Madge Framelogger.

So remember, the end station indicates source routing by setting the high order bit of ) the source address. There is no special bit reserved solely for this purpose. Routing Information Field (RIF)

______Student Notes

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HDR DA SA RIF LLC DATA Trailer

2bytes Maximum of 18 bytes Routing Information Control 001 002 003 Field A B 0 Ring Ring Ring Bridge Bridge Bridge 2 bytes minimum – Control information only 18 bytes maximum: Ring + Bridge pairs – ie maximum hop count (7 bridges ) reached Last bridge number always 0 – destination node is on a ring

In fact when the station sets the RIF indicator flag it also sets up a 2 byte control field. This field contains, amongst other things, the exploration strategy to be used by the station when looking for the server. The control field is something we will discuss in much more detail later in the document.

You can see from its position in the MAC header of the data frame that the Routing Information Field contains information which allows LLC to carry data (possibly other protocols) over bridges. This makes source routing a layer 2 protocol.

Source route bridging actually follows one of two protocols: IBM or IEEE. The key difference between the two is the maximum hop count, 7 in the case of IBM, 13 in the case of IEEE. This means that on an IBM network the frame will be discarded if a bridge notices that a frame has already crossed 7 bridges.

6.3 Self study - Draw a frame with the maximum number of hops

To understand how RIFs are built up it is useful to consider the maximum size of RIF that is possible on an IBM network.

First, there are 2 bytes of control information. A ring number requires 3 hex digits (1 1/2 bytes) and a bridge number requires 1 hex digit (1/2 byte) so a ring/bridge pair require 2 bytes. This means the simplest possible RIF would be:

Control Ring Bridge .. header bytes ______Student Notes

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2 2 bytes 001 A 002 0

Don’t forget the final bridge number, 0, which is only there to terminate the RIF. The search for a server must end on a ring.

Right, its time for you to try drawing a RIF with the maximum of 7 hops. Imagine a network with 7 bridges and the appropriate number of rings.

Once you have drawn this count up the total number of bytes for this RIF.

Then you can turn the page to see if you agree with the “official answer”.

______Student Notes

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6.3.1 Answers

Here is an example RIF which has reached maximum length. Control 2 bytes Header Ring/Bridg e 2 bytes 001 A 00 B 003 C 004 D 005 E 006 F 007 1 008 0 2

This RIF is 18 bytes in length and consists of 8 ring/bridge pairs allowing a total of 7 bridge hops. The last bridge number is null as the frame must land finally on a ring. Constantly Circulating Frames Unique Ring Numbers

Control 002 1 004 2 002 0 Ring 004 Ring number not allowed to be in RIF more than once 1 2 3 Stops constantly circulating frames Ring 002

One final point about how RIFs are constructed.

We have already made it clear that ring numbers must be unique. A bridge always expects to see a particular ring number only once in any RIF. If it recognises that the ring number of its output ring (002 in the example above) is in the RIF it will not write the Ring Number into the RIF again; it will discard the frame. The purpose of all this is to prevent frames constantly circulating the network. This is vital on switched networks where loops are deliberately introduced to provide alternative paths and thereby load balancing.

______Student Notes

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6.4 Test your understanding: Theory of Source Routing

1. In a token ring frame the Routing Information Field (RIF) is positioned a) before the MAC address b) after the destination MAC address and before the source MAC address c) after the source MAC address and before the data d) after the data

3. The control header in the RIF is: a) always exactly 2 bytes long b) at least 2 bytes long c) optional

4. Two source routing bridges with the same bridge number must not a) exist on the same network b) attach to the same ring c) join the same two rings

______Student Notes

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7. Theory of Source Route Bridging: Explorer Frames

Source Route Bridging All Routes Explorer (ARE) Frames

ARE 2 2

0013 0023 003

A B

1011 102 1 103

We can now apply the basic knowledge we learned in the previous chapter. In the diagram you can see a perfectly valid resilient network with multiple paths from the source end station to the destination end station.

The destination end station could be a server (as pictured) but it could equally be a mainframe or another workstation.

Ring numbers are unique, bridge numbers are unique where necessary, that is when joining the same two rings e.g. 001 and 002.

The end station wishes to start a session with the destination and sends an explorer frame to find it. It sets the RIF indicator and also sets up the first 2 bytes of the Control header. In this header it indicates the exploration strategy.

This is the strategy requested by the source route bridging end station to explore the network and find the end station. This is fixed in the first 3 bits of the Control header at the start of the RIF.

Over the next few pages we will be looking at the different frame types needed for ) source routing. The first examples will be general. Later, we look at how these frames are used in specific manufacturer environments for example Novell.

______Student Notes

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First we will investigate a common strategy which is to explore all routes round the network to reach the server. This uses an All Routes Explorer frame or ARE. Source Route Bridging All Routes Explorer (ARE) Frames

2 2 0013 0023 003

A B

1011 102 1 103

This is the result once the ARE has crossed onto rings 101 and 002. The frame will be copied by bridge 2 ,bridge 3 and bridge A.

7.1 Self study: All Routes Explorers

1. Given that the frame will go in both directions (via 101 and 002) can you determine how many copies will end up on the destination ring?

Answer:______

2. How many copies of the frame will be seen on ring 101 (not necessarily at the same time)?

Answer:______

Source Route Bridging All Routes Explorers

______Student Notes

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ARE: 001-2-002-2-003-0 2 2 ARE: 001-3-002-2-003-0 ARE: 001-3-002-2-003-0 0013 0023 003 2 2 ARE: 001-2-002-3-003-0 001 3 002 3 ARE: 001-3-002-3-003-0 A B

ARE: 001-2-002-2-003-B-103-0 A B ARE: 001-3-002-2-003-B-103-0 1011 102 1 ARE: 001-2-002-3-003-B-103-0

101 1 102 1 103 ARE: 001-3-002-3-003-B-103-0 ARE: 001-A-101-1-102-0 ARE: 001-A-101-1-102-1-103-0

The answer to both questions is 5. The explorer frames will reach the destination ring from both directions, 4 frames via the top route, 1 via the bottom route.

The 4 frames will carry on round from ring 003 and reach 101. Ring 101 will already have seen the same explorer frame coming via bridge A, so the total for 101 is also 5.

7.2 Why use All Routes Explorers?

With AREs it is possible (as in our case) for multiple copies of the ARE to arrive at the destination. The destination will respond to each of these with a specifically routed frame which will follow the route given in the ARE’s original RIF. This means the specifically routed frame will go back the way the ARE came.

Eventually a number of specifically routed frames will get back to the source, and the source will store in the cache the first route it receives.

) 7.3 Disadvantages of All Routes Explorers

With All Routes Explorers numerous copies of the request can arrive at the destination and this is not always desirable. This whole process allows the source station to determine the best route by taking the path with the shortest round trip delay. This is a key principle in the theory of source route bridging.

For this reason an alternative strategy is to use a single route explorer to reach the destination.

______Student Notes

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Clearly there needs to be a way to provide a single route to the destination in first place. The end station needs plenty of help from the bridges to do this. A technique called Spanning Tree is used to provide this. The term Spanning Tree is used to refer to a network that has only one path to get between any pair or rings.

______Student Notes

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Source Route Bridging Example Spanning Tree

= Standby Bridge

2 2

0013 0023 003

A B

1011 102 1 103

Only one route from any ring to any other ring

From the diagram it is clear that something has determined that certain bridges must be “turned off” these are both bridges numbered 3 and also bridge 102-1-103. The bridges remaining active are called designated bridges. This means that these are the only bridges that will pass Spanning Tree Explorer frames. Simply expressed, this means that the Spanning Tree Explorer frames will traverse the designated bridges to explore the route to the destination station.

Note: Do not get confused with transparent bridges. A designated bridge under ) transparent bridging will carry data frames. The spanning tree is there to provide a loop free path for data itself. There is no concept of explorer frames. This means that a standby bridge is basically inactive. It only becomes active when it has to take over from the active bridge in case of failure.

7.4 Why use Spanning Tree Explorers?

The whole point of this is that there is only one route to the server, one route from one ring to any other ring. This means only one copy of an STE frame appears on each ring but the destination station is still found. ______Student Notes

Copy of Sg_trn02A  Madge Networks Page 48 of 93 Madge Training TRN-03 - Multisegment Token Ring ______

Source Route Bridging Specifically Routed Frame SR: 001-2-002-2-003-0 SR: 001-2-002-2-003-0 2 2 2 2 0013 0023 003 0013 0023 003

A B A B

1011 102 1 1011 102 1 103 103

SR: 001-2-002-2-003-0

2 2

0013 0023 003

A B

1011 102 1 103

Let’s just review the basic process again:

A Specifically Routed Frame is sent in response to each ARE or STE received by the destination Note, the RIF remains unaltered, but a single bit, the direction bit, is altered. This ) tells the bridges en route to read the RIF backwards.

In the case of AREs one or more specifically routed frames will be received by the source station. It will then use the RIF in the first specifically routed frame it receives for its new session with the destination. If it sent an STE out the return frame will also follow the spanning tree and that is what the source station will use for its session with the destination.

______Student Notes

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Source Route Bridging Exploration Strategies Header DA SA RI LLC Data Trailer Header

Control 0-8 Ring & Bridge Number fields

3 bits 5 bits 1 bit 3 bits Broadcast Length of Maximum Routing Info Direction Unused Indicator (in bytes) bit frame size

000 Specifically Routed Frame 000: 516 bytes 100: 8144 bytes 100 All Routes Explorer (SR return) 001: 1500 bytes 101: 11407 bytes 110 Spanning Tree Explorer (ARE return) 010: 2052 bytes 110: 17800 bytes 111 Spanning Tree Explorer (SR return) 011: 4472 bytes 111: Initial value

7.5 Exploration Strategies

Note: all reference books call the first 3 bits of the control field the “Broadcast Indicator”. We will always refer to them as providing the “Exploration Strategy”.

The options which can be set by the source station driver software determine the strategy which the frames will follow when crossing the bridges to the destination station.

This then gives the complete picture of how end station interact when trying to find a ) route for a new session. 3 bit Indicator Exploration Strategy code 000 Specifically Routed Exploration is complete, use the specified Frame route 100 All Routes Explorer (SR Explore All Routes from the source, the return): destination replies, the source caches the RIF in the first SR frame it gets back

______Student Notes

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3 bit Indicator Exploration Strategy code 110 - Spanning Tree Out, All Possibly the ideal strategy, find the Routes Explorer Return destination with an STE, then discover the best route on the way back. This allows load balancing. 111 Spanning tree out, use This option uses no AREs, but means only SR (the same route) one path is used. This means no load back balancing, so it takes away one advantage of spanning tree.

7.6 Pros and cons of ARE out SR return

The main benefit of this strategy is that once the source station has received its first response to its ARE out it knows it has the fastest route. Subsequent explorations may provide a different station with a different route. This is the benefit of the round trip calculation the strategy provides.

In this case the destination (normally a server of some kind) receives multiple copies and must process them all. Each bridge and ring can become overloaded if there are many alternate paths.

7.6.1 Broadcasts using All Routes Explorers

An All Routes Explorer can be sent as a unicast if the network address (MAC address) of the server or end station is known. But what if the MAC address is not known? Then we must send a broadcast.

This would be the case if the source station is a Novell workstation wishing to start a session with a Novell server. The workstation would send an IPX SAP request otherwise known as a Get Nearest Server request.

Every bridge will copy every ARE it sees. The AREs sent by the station are broadcast frames and will be processed by every station on every ring on which that ARE is seen. This includes all the servers which should not be spending valuable CPU time processing broadcast frames not intended for them.

______Student Notes

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7.7 Pros and cons of STE out ARE back

The benefit compared to the first strategy (ARE out) is that the end station (normally a server) only receives one frame and is itself responsible for sending out AREs. This means load balancing can be combined with a lower processing overhead on the server , bridges and other stations. However the STE is often sent as a MAC broadcast frame.

7.7.1 Broadcasts using Spanning Tree Explorers

An STE broadcast must still be processed by all stations on the ring, but the overhead is reduced since only one copy appears on each ring.

7.8 Pros and cons of STE out SR back

The advantage of this is that the load on bridges, rings, destinations and other stations is reduced. But no load balancing is provided.

7.9 Control Header Fields

After the 3 bit broadcast indicator you see the following fields:

Length of routing information - very useful if you are a bridge wishing to skip forward to the start of LLC data which would allow you to perform filtering

Direction bit - tells the bridge in which direction to read the RIF. Once the server has decided which RIF to use it will start to use Specifically Routed frames. To do this it must change the direction bit to tell the bridge not to read the RIF left to right (from the station), but right to left (from the server).

Maximum frame size - rarely used by drivers. Exploration Example Spanning Tree Out with All Routes Explorer Return

______Student Notes

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ARE: 003-2-002-2-001-0

ARE: 003-3-002-2-001-0

ARE: 003-2-002-3-001-0

ARE: 003-3-002-3-001-0 2 2 STE: 001-2-002-0 0013 0023 003 2 2

0013 0023 003 A B

A B

1011 102 1 103 ARE: 003-B-103-1-102-1-101-1-001-0 1011 102 1 103

STE: 001-A-101-0 Station chooses best RIF, then uses SR frames

Let’s look at what might be considered the ideal exploration strategy: STE out, ARE return.

Let’s do this by going through the procedure followed by the end station which wants to find a server using source routing. This time let’s consider a Novell workstation:

1. Driver software is loaded on the end station to ensure the RIF indicator flag (bit 1 of the source address) will be set. This is ROUTE.COM for a workstation, but equally ROUTE.NLM carries out the same job on a Novell server. ROUTE.COM must also set the broadcast indicator, in this case to 110: STE out, ARE return.

2. Once the IPX stack has been loaded, requester software called NETX.COM (or something similar in the case of VLMs or Client32) is loaded. This sends out a SAP request for a server. If the Preferred Server option was included then a response from a named server will be expected.

3. The exploration strategy will now be followed - with one large proviso: the server is ) at liberty to ignore the strategy requested by the end station. For example, certain older versions of ROUTE.NLM ignore requests to perform AREs back to the workstation.

The STE will take two routes in order to reach all rings. Only one explorer will arrive at the server.

The server can then use a unicast All Routes Explorer to find the best path back to the client. Note that this is a unicast as the MAC address of the client is known.

______Student Notes

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7.9.1 The ideal exploration strategy?

The full description of the exploration strategy used here is Spanning Tree Explorer Out, All Routes Explorer Return then Specifically Routed.

The station finds its server with an STE; the server explores the route back with a unicast ARE; the end station chooses the “best” RIF from those it receives and uses the specified route to return to the server. This is the route it uses for the duration of the session. In the case of a Novell workstation, this is until the user unloads NETX. When NETX is reloaded a new STE will be transmitted (if this is the chosen exploration strategy). The detail of all this is going to become clear when you try this in the practical session which is coming up after the next chapter.

______Student Notes

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Source Route Bridging Bridge Decision Process: 3 Logical Bridges All Routes Explorer Yes Always Forward, Source unless: Routed frame? • AREBridge Rules broken Check RIF Spanning Tree Explorer No Forward if bridge configured for STEs, unless: • STE Bridge Rules broken

Ignore frame Specifically Routed Frame Forward if RIF ring & bridge ARE Bridge Rules numbers match Never forward if STE Bridge Rules • already been on next ring unless Never forward if • hop count exceeded • Bridge Rules broken • already been on next ring • hop count can be less than • max hop count of 7 exceeded max of 7 ie configurable • not configurable to less • filtered (MAC address etc) •filtered (MAC address etc)

7.10 Bridge Decision Process

The flow chart above summarises the possible roles played by each source routing bridge. You can even think of this process in terms of three logical bridges within the physical bridge. Each bridge deals with a different type of source routed frame: all routes explorer; spanning tree explorer; specifically routed.

7.11 Review of Exploration Strategies: a real world issue

The picture overleaf summarises the key points you need to “take home” on Source Route Bridging Exploration Strategies.

The key point to remember is that each strategy has strengths and weaknesses. Factors to consider include: need for load balancing, level of AREs; difficulty of applying uniform strategy with differing platforms and driver versions.

______Student Notes

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Source Route Bridging Exploration Strategies

OUT BACK ARE SR STE ARE STE SR

Each strategy has strengths and weaknesses

Full route discovery means using AREs – otherwise no load balancing After route discovery, ALL subsequent frames are Specifically Routed

Log transactions to identify ACTUAL behaviour

– server might not follow request of client

If load balancing is a pre-requisite then AREs must be used at some point.

The last, and perhaps the most important point to remember about designing source routed networks is as follows.

Clients, servers, mainframes and gateways might not obey the rules described above. The real world is more complex. This means that as an engineer you must become fully acquainted with ACTUAL behaviour by logging. That’s precisely why in the practical session we will start this learning process using Madge’s own Framelogger.

______Student Notes

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7.12 Test Your Understanding: Explorer Frames

1. If source route bridging is implemented, bridges joined in parallel can provide: a) alternative routes for traffic b) load balancing c) contingency in case of failure d) all of the above

2. If transparent bridging is implemented, bridges joined in parallel can provide: a) alternative routes for traffic b) load balancing c) contingency in case of failure d) all of the above

3. Specifically routed frames are copied by every source routing bridge. a) True b) False

4. Spanning Tree Explorer frames are copied by every source routing bridge. a) True b) False

5. All Routes Explorer frames are copied by every source routing bridge. a) True b) False

6. When applied to source routing bridges the spanning tree protocol provides a path for a) single route explorer frames only b) single route and all route explorer frames only c) non-explorer frames only

7. The Spanning Tree Protocol is recognised a) by bridges only b) end stations only c) bridges and end stations

8. Spanning Tree ______Student Notes

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By the end of the session you will be able to... Identify the steps taken by the Spanning Tree algorithm to nominate: – the Root bridge – Designated bridges – Standby bridges Explain the parameters which can be configured to influence the Spanning Tree

Describe the key fields of the Spanning Tree frame

Explain how Spanning Tree eliminates loops

– providing a single route between any two rings

The purpose of Spanning Tree when used with Token Ring can be described in very simple terms. The Spanning Tree on its own can be applied to any network topology and is guaranteed to provide a network without loops, where there is only ever one route from one station to any other. In terms of Token Ring this means there is a way for a frame to be sent out so that it reaches its destination using only one route. The result of that is that only one frame arrives at the destination.

To achieve this state each bridge has to establish whether it should forward frames or not, i.e. should it be forwarding or blocking. Its mode while it is establishing this is called learning mode; it is learning the topology of the network to identify its role.

In fact, the bridges must co-operate using some sort of protocol to ensure that certain bridges are switched off to eliminate loops. With Token Ring we can use the analogy suggested earlier. A source routing bridge contains three logical bridges for:

1. All Routes Explorers 2. Spanning Tree Explorers 3. Specifically Routed Frames

Spanning Tree can effectively “switch off” the second of these two logical bridges. In other words the Spanning Tree Algorithm cannot affect the passing of AREs and SR frames. These will always be passed unless the bridge rules (hop count etc.) will be broken in doing so.

Example Spanning Tree ______Student Notes

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Root (Designated) Ring 101 1 Ring 102

Designated 3 2 Designated

4 Ring 103 Ring 104 Standby

Path from 103 to 104, discovered by an STE frame, is via 101 and 102

Here is an example Spanning Tree consisting of bridges 1, 2 and 3. An STE frame sent by a station wishing to start a session with a server on 104 must follow the pre- determined Spanning Tree. Bridge 4 is in standby but will forward AREs and SR frames.

A single Root bridge is the centre of the spanning tree. Other bridges will measure the cost of sending frames to the root bridge. Only bridges on the cheapest route will be enabled by the protocol for forwarding STEs.

For rings 103 and 104 the cheapest bridges to use are bridges 3 and 4. These become the designated bridges. The root bridge is always designated.

But how does the Root Bridge get chosen in the first place?

Spanning Tree Formation Election of the Root Bridge

______Student Notes

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Each bridge has a Bridge Label – 4 hex digits (8000h or C000h) Each bridge has a number of MAC Addresses – 12 hex digits (0000F6123456) – read in non-canonical (MSB first) Bridge ID is Bridge Label + Mac Address – (8000)(00006F482C6A) – MAC Address read in Canonical (LSB first)

Lowest Bridge ID becomes Root Bridge

8.1 Addressing formats for Spanning Tree frames

Before the discussion on Root bridge election we need to clarify the issue of address formats.

Because Spanning Tree is a standard originated from the world of Ethernet, it uses Ethernet-style addressing. This means that addresses in frames are held in canonical format.

If you are not aware of canonical and non-canonical addressing then you should read the next section and complete the exercise.

If you do understand canonical and non-canonical addressing then skip to the section Election of Root Bridge - the Bridge Id

8.2 Canonical and non-canonical addressing formats

A change of network type means an even greater overhead for the bridge as address translation may be required. Ethernet and FDDI use canonical addressing. This means the bits in an address byte are read from right to left, or from the least significant to the most significant. Token Ring uses non-canonical addressing, i.e. addresses are read most significant bit (MSB) first. So when a token ring station needs to talk to an FDDI station across a bridge the bridge must perform the translation.

Remember, the contents of the address field are not changed by this process. It is merely a matter of reading a byte from the left most bit (high order or MSB) which is ______Student Notes

Copy of Sg_trn02A  Madge Networks Page 60 of 93 Madge Training TRN-03 - Multisegment Token Ring ______the non-canonical way, or from the right most bit (low order or LSB) which is the canonical way.

Incidentally 00 00 F6 as the first 3 bytes of a MAC address specifies a Madge Token Ring Adapter. This can be easily identified in canonical format as you just swap the hex characters round.

Analysis of a single Token Ring address byte, binary and hex Expressed as a byte: Expressed in bits Same bits Expressed as a byte: Non-canonical Read MSB to LSB Read LSB to MSB Canonical i.e. Canonical F6 MSB11110110LSB LSB01101111MSB 6F

E6 MSB11100110LSB LSB01100111MSB 67

Test your powers of bit manipulation now by filling the gap in the following table. Turn to the following page for the answers. Analysis of a single Token Ring address byte, binary and hex Expressed as a byte: Expressed in bits Same bits Expressed as a Non-canonical Read MSB to LSB Read LSB to MSB byte: i.e. Canonical Canonical F7 11110111

10000110 61

______Student Notes

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8.2.1 Answers to questions on canonical and non-canonical addressing

Non-canonical Non-canonical Canonical Canonical

F7 11110111 11101111 EF

86 10000110 01100001 61

8.3 Election of Root Bridge - the Bridge Id

Each bridge has a 4hex digit value assigned to it called the Bridge Label. This appears in hex when you log the frame using Framelogger, and in decimal when you configure the value using Madge Trueview Bridge manager.

Another difference is that under Trueview Bridge manager the Bridge Label is called the Bridge Priority.

Manufacturers assign default values to the Bridge Label, 8000h and C000h being two possible defaults.

Each bridge port has a MAC address assigned to it. A multi-port bridge or switch might have 4 or more MAC addresses, normally assigned sequentially. These are read as normal MAC addresses i.e. non-canonically or Most Significant Bit first (LSB)

Spanning tree takes the Bridge Label and appends the lowest MAC address to it to form the Bridge Id. The Spanning Tree Protocol will run allowing all bridges to exchange Bridge Ids. There is guaranteed to be a single Bridge Id which is the lowest and this will become the root bridge. Bridge Ids are read canonically or Least Significant Bit first (LSB).

______Student Notes

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Spanning Tree Formation HELLO Bridge Protocol Data Unit (BPDU)

SDEL AC FC DA SA RI DATA FCS EDEL FS

LLC header HELLO BPDU

42h 42h UI 3 bytes DSAP SSAP Control

Root Root IDPath Bridge ID 35 bytes Cost

Flags Forward Delay BPDU Type Hello Time Protocol Version ID Max Age Protocol Identifier Message Age Port Identifier

The exchange of Bridge Ids happens during a so-called Election process during which each bridge assumes it is the only bridge on the network and sends out a frame announcing itself as the root.

This frame is called the Bridge Protocol Data Unit or HELLO frame.

These frames are sent to a specific functional address which means they are not broadcast frames. This is important, as it means workstations will not be burdened with the large quantity of these frames sent out during the root election. " Once the election is over we will see that the bridge with the lowest Bridge ID actually becomes the root.

We are not concerned with all the detail of the frame. We will look at Root Id, Root Path Cost and Bridge ID. During the Root Election each bridge will send out BPDU frames with its own Bridge Id in both the Bridge Id and the Root Id fields. It assumes initially that it should be the Root Bridge. However it will also receive BPDU frames from other bridges. When it sees a BPDU with a Root ID that is lower than its own it ______Student Notes

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stops sending out BPDUs on that specific ring. It then prepares its own BPDU with the Root Id it has just discovered and sends these BPDUs away from the (potential) root bridge. In turn this Root Id will be compared to those sent by neighbouring bridges until all l Root Ids have been compared. Eventually, all bridges will be sending BPDUs with the same Root Id. In other words they agree which bridge should be the root bridge: the bridge with the lowest Bridge Id.

Spanning Tree Formation Election of Root Bridge 100000006fabcdef Hello BPDU

Ring 101 1 Ring 102

Hello BPDU Hello BPDU 3 2 100008005aabcdef 800000006f123456

Ring 103 4 Ring 104

Hello BPDU 800000006f654321 The diagram shows this process clearly. All bridges will start sending BPDUs on all ports. Each bridge will examine the Bridge Ids in the BPDUs it receives. Eventually only one bridge per ring send BPDUs, all with the same Root Id.

What is the Root Id of the root bridge?

______(Answer is over the page).

Warning: the Bridge Id is nothing to do with the Bridge Number. It’s the bridge ) number that gets written into the RIF as the RIF is being built up. The Bridge Id is used solely for determining the Spanning Tree.

______Student Notes

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Answer to quiz: 100000006Fabcdef i.e. Bridge 1

8.4 Determining the designated bridges - Root Path Cost

There is a standard spanning tree parameter associated with each bridge called the Path Cost.

This term refers to a parameter within the bridge that determines how suitable a bridge is to be included in the spanning tree. Generally slower bridges should be given higher path costs so that they are only used if a faster bridge is unavailable.

The Root Path Cost is the sum of the Path Costs of the bridges between a node (or ring) and the root bridge. The Spanning Tree Protocol uses this to decide which path to use if there is more than one path between the node and the root bridge. By altering the Path Costs on bridges you can alter the relative Root Path Costs and hence manage the topology of the spanning tree.

To give an example, you will later observe the Root Path Cost recorded in the BPDU. BPDUs from the Root Bridge contain a Root Path Cost of 0 (zero). This is because the cost to reach the root bridge is 0 i.e. there are no bridge hops. All other bridges will transmit frames with a non-0 value for the Root Path Cost in the BPDU.

______Student Notes

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Spanning Tree Example

8000 00006f654321 Ring 101 1 Ring 102 5

8000 8000 1000 00006f123456 00006fabc123 08005aabcdef 3 4 2 10 15 20

15 Ring 103 5 Ring 104 1000 00006fabcdef Identify the Root and Designated bridges Explain Why & How

8.5 Self-study: predicting the Spanning Tree

Look at the above diagram and answer the following questions:

1. Which bridge will become the root bridge 2. Calculate the Root Path Costs from each ring and determine which bridge(s) will become standby. 3. Explain your reasoning.

Answers are over the page.

______Student Notes

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Answers to self-study: 1 00006fabcdef (Bridge number 5) is the lowest Bridge Id and will become the Root. 2 Bridges 1 and 3 will become designated. Bridge 5 is the root, so must be designated. 3 Bridge 3 has a cost of 10 to reach the root bridge from 101 so make this designated Bridge 1 has a cost of 15 (5 + 10) to reach the root bridge via bridge 3, less than the cost of bridge 2 (20) Bridge 5 is the root, so must be designated Spanning Tree Formation Maintenance of Spanning Tree Hello BPDU

Root Ring 101 1 Ring 102

3 Designated 2 Bridges Hello BPDU

Ring 103 4 Ring 104 Standby Bridge

This is what a Spanning Tree will look like when it has stabilised. The root bridge has been determined, root path costs to it from each ring have been calculated so the designated bridges have been determined. The root bridge has two designated ports and sends out BPDUs on each. Bridge 2 sees the root bridge’s BPDU from ring 102, bridge 3 sees them from ring 101. Bridges 2 and 3 will send BPDUs on to the rings for which they are designated, as shown above that is rings 104 and 103 respectively. As long as bridge 4 continues to see BPDUs it remains standby. If it misses 3 of them from any ring it will take over as designated.

8.6 Review Spanning Tree

• The term Spanning Tree is used to refer to a network that has only one path to get between any pair of rings.

______Student Notes

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• The Spanning Tree is used by frames needing to explore a single route to the server - Spanning Tree Explorer frames (STEs). • Spanning tree is calculated by first identifying a root bridge. This is the bridge with the lowest Bridge Id (Bridge label + MAC address in canonical format) • Designated bridges are identified as those bridges which form part of the lowest cost path from a ring to the root bridge. These will pass STEs • The Spanning Tree Protocol is implemented using HELLO Bridge Protocol Data Units which are sent to the Bridge Functional Address by each bridge. Each BPDU contains the Bridge Id, the Root Id and the Root Path Cost plus other detailed information.

8.7 Test your knowledge: Spanning Tree

1. The Root bridge is the bridge with the a) highest Bridge Id b) highest Bridge Number c) lowest Bridge Id d) lowest Bridge Number

2. The main way to force a bridge to be designated a) lower the Root Path Cost b) lower the Path Cost c) lower the Bridge ID d) lower the Bridge Priority

______Student Notes

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9. Ring and Bridge Number Configuration Workshop By the end of this session you will be able to... Use Trueview Bridgemanager to: – assign ring numbers – assign bridge numbers Use Trueview Bridgemanager and the LCD display to: – Troubleshoot configuration errors – Correct configuration errors

9.1 Pre-requisites for this workshop

• You have the equipment listed below • You have passed all the “Test your understanding” tests on Source Route Bridging and Spanning Tree. • You have Trueview Bridgemanager installed on your PC • You are familiar with basic Trueview operations

9.2 Practical Objective

You will need to work with a partner to set up the following network. This network must be functioning correctly before you proceed to the Spanning Tree Workshop.

______Student Notes

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Novell Server Backbone Ring (124)

Ring FFF Bridge D Ring 123 Bridge A Ring 123 Ring FFF Bridge B Bridge C

9.3 Points to remember

1. This networks requires

a) 4 bridges b) 5 hubs c) only 3 token rings in other words, the network has a backbone ring (124) with connections to 2 other rings via parallel bridges.

2. The bridge will perform “Bridge Tests” whenever ring numbers and bridge numbers are re-configured. These are displayed on the LCD panel and must be successful, otherwise they indicate configuration conflicts.

9.4 Instructions to follow

There is no pre-set way of doing this. Please draw a logical diagram of the network if you are unsure what to configure first.

9.5 You will have completed this exercise when...

1. The correct ring numbers and bridge numbers have been assigned 2. No Bridge tests are occurring

______Student Notes

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3. You can connect to the Novell server using NETX.COM

______Student Notes

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10. Spanning Tree Workshop By the end of this session you will be able to...

Use Framelogger and the LCD display to: – determine which bridges will pass STEs – show how the Bridge Id is used for root election – explain how Path Costs are used to determine which bridges will be designated

10.1 Pre-requisites for this workshop

• You have set up the network above with parallel rings • You have Framelogger “ installed on your PC and know how to use it. • Framelogger should be attached to your user ring i.e. 123 or FFF.

10.2 Practical Objective

You need to examine Hello BPDUs on different rings, transmitted by the root bridge and by other bridges.

10.3 Points to remember

You are looking at Hello BPDUs which are LLC frames which continue to be transmitted by all designated bridges after the root election has completed.

______Student Notes

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10.4 Instructions to follow

1. Using only Framelogger, determine which of the four bridges will pass Spanning Tree Explorer frames. The questions further down will lead you to the answer.

2. On a the Framelogger workstation start Framelogger by typing:

CD\TOOLS\FL2 FRLOG

3. Configure Framelogger to collect the whole of LLC frames only.

4. Take a log for about 20 seconds

5. What is another name for SPT frames? What is indicated by the presence of SPT frames?

______

6. There are 5 levels of decode on Framelogger, use level 5 to look at the SPT frames and obtain the following information:

- What is the node address of the root bridge?

______- What is the node address of the bridge sending this SPT frame?

______

- Is this bridge the root bridge?

______

- What is the path cost to the root bridge?

______

- What is the bridge label of this bridge?

______

7. - From your diagram, how many bridges are on this RING (not network)?

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- How many bridges are sending SPT frames?

- Why is the bridge which is sending the SPT frames configured to pass Spanning Tree Explorer frames?

HINT: If you had a number of bridges in parallel, how many would be configured to pass Spanning Tree Explorer frames, and therefore how many would you want to be sending the SPT frames?

______

8. You will have deduced by now whether you are looking at Hello BPDUs from the root bridge. If you are not, repeat point 6 and compare the Root Path Cost with your original result. What is your conclusion?

______

10.5 You will have completed this exercise when... you have filled in all the blanks above and you understand how the root and designated bridges were selected. You will have understood the role of the Bridge Id and the Root Path Cost. Don’t forget to check your answers overleaf.

______Student Notes

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10.6 Spanning Tree Workshop: Answers

5. “SPT frames”, Spanning Tree frames is the term used by Framelogger to describe Hello BPDUs. These frames indicated that a bridge designated to forward these frames is present on the ring, and that Spanning Tree is active.

6. What is the node address of the root bridge? This will start 0000F6 and will be a Madge 6 byte MAC address What is the node address of the bridge sending this SPT frame? This will be the same as the address above if the root bridge is on your ring. Is this bridge the root bridge? Yes, if the above is the case What is the path cost to the root bridge? If the root bridge is on your ring, then the root path cost is zero, i.e. there are no hops required to reach this bridge What is the bridge label of this bridge? The bridge label on a Madge bridge defaults to 32K or 8000h.

7. On any single ring only the designated bridge for that ring will be sending Hello BPDUs.

______Student Notes

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11. Explorer frames Workshop By the end of this session you will be able to...

Use Framelogger to: – analyse the broadcast indicator bits of the RIF – observe explorer frames being sent – determine whether the exploration strategy requested has been carried out – Explain the difference between OUT BACK ARE SR STE ARE STE SR

11.1 Pre-requisites for this workshop

• You have completed the Spanning Tree Workshop

11.2 Practical Objective

You need to become aware that even if a workstation requests a certain exploration strategy the server may reject this. This is dependent on the end station software such as ROUTE.NLM.

11.3 Points to remember

When you are decoding the Control header of 2 bytes you will see 4 hex digits.

One example would be C660 which decodes as follows

______Student Notes

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C 6 6 0 1100 011 011 0000 0 0

The first 3 bits (emboldened) tell us this is an STE. Other possible values are as follows.

Broadcast 000 100 110 111 indicator Broadcast Specifically All Routes Spanning Spanning strategy Routed Frame Explorer Tree Explorer Tree Explorer (ARE return) (SR return)

11.4 Instructions to follow

1. Use Framelogger to log a test PC sending frames to a NetWare server across the bridge. Start the test PC by typing the following:

CD\MADGE SMART IPX SR=Y NETX

NB: You only need to start the log just before you type the NETX statement.

2. View the log. Look for the first IPX frame sent from your test PC.

- What is the value of the Control Field in the Routing Information Field?

______

- What exploration strategy is being requested?

______

- Is it Forward or Backward (Direction bit set to 0 or 1)?

______

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3. Look at the first IPX frame returned to your test PC.

- What is the value of the Control Header in the Routing Information Field?

______

- Is this a Limited (STE), General (ARE) or Specifically Routed frame? ______

- What route was taken by this frame (in the correct direction, please)?

______

4. Therefore, what Exploration Strategy was used?

______

This is not the requested broadcast strategy. We now propose a way of ensuring that an All Routes Explorer frame is sent out.

5. Repeat the previous steps, first rebooting the test PC and enabling IPX as follows:

CD\MADGE SMART IPX SR=Y GBR (This stands for General Broadcast) NETX

NB: You only need to start the log just before you type the NETX statement.

6. View the log. Look for the first IPX frame sent from your test PC. Is this a Limited (STE), General (ARE) or Specifically Routed frame?

______Student Notes

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______

7. Look at the IPX frames returned to your test PC.

- What is the value of the Control Header in the Routing Information Field?

______

- Are they Limited (STE), General (ARE) or Specifically Routed frames?

______

- What route was taken by these frames (in the correct direction, please)?

______

9. Therefore, what Exploration Strategy was used?

______

______Student Notes

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11.5 Answers to Explorer Workshop ) In this first part you should have observed how the station requests STE out, ARE back. In fact the server responds with a specifically routed frame, so the strategy becomes STE our SR back, an incorrect response from the server. 2. View the log. Look for the first IPX frame sent from your test PC.

- What is the value of the Control Field in the Routing Information Field?

C270

- What exploration strategy is being requested?

Limited (STE) out, ARE back (its a SAP request)

- Is it Forward or Backward (Direction bit set to 0 or 1)?

Forward

3. Look at the first IPX frame returned to your test PC.

- What is the value of the Control Header in the Routing Information Field?

06E0

- Is this a Limited (STE), General (ARE) or Specifically Routed frame?

Specifically routed

- What route was taken by this frame (in the correct direction, please)?

Read the RIF backwards from the server ring i.e. 124 to FFF or 123

4. Therefore, what Exploration Strategy was used?

Spanning Tree Out, Specifically Routed back

______Student Notes

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Conclusion ) The point to be made here is that TWO AREs should come back over the two bridge links. This would mean exploration is taking place.

In fact the strategy has become STE our SR back, an incorrect response from the server. To remedy this we can force the outgoing frame to be an ARE.

So you reran the Smart IPX with GBR which sends a General Broadcast (ARE).

11. View the log. Look for the first IPX frame sent from your test PC. Is this a Limited (STE), General (ARE) or Specifically Routed frame?

ARE out (8270h)

12. Look at the IPX frames returned to your test PC.

- What is the value of the Control Header in the Routing Information Field?

06E0

- Are they Limited (STE), General (ARE) or Specifically Routed frames?

Specifically routed

- What route was taken by these frames (in the correct direction, please)?

Read the RIF backwards from the server ring

13. Therefore, what Exploration Strategy was used?

ARE out, Specifically routed back

11.6 Explorer Workshop conclusions

The unexpected behaviour of the server is controlled by ROUTE.NLM which has been chosen deliberately for this reason. The following extract (reproduced in full at the end of the guide) explains how Novell have altered the default values for explorer frames.

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Current versions of ROUTE.NLM do obey the strategy requested by the workstation (STE out/ARE back).

“NOTE: Prior to the 1994 release of the source route server end station software, the behaviour was different. In these earlier releases, the source route server software responded with a Specifically Routed Explorer Frame and could also be configured to respond to an explorer frame with an All Routes Explorer Frame”

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12. Appendix- Test your understanding: Answers

12.1 Bridging Fundamentals

1. What type of device is hidden from layer 3 protocols like IPX? a) A repeater b) A transparent bridge c) A source route bridge d) All of the above

2. A router works at which layer of the OSI model a) Physical b) Datalink c) Network d) Transport

4. A device which copies frames based on the source and destination MAC address is called a) a repeater b) a router c) a transparent bridge d) a source routing bridge

5. Why is the following RIF invalid? Ring No. Bridge No. Ring No. Bridge No. Ring No. FFF 1 00B 3 0 A RIF must end with a 0 bridge number as it must end up on a ring.

7. In a Token Ring frame the RIF is

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a) present if requested by the bridge b) present if requested by end station software c) optional if the RIF flag is set d) mandatory

8. A transparent bridge acquires addresses and assigns each address to one of its ports. It does this by a) Examining the source and destination address b) Examining the source address only c) Examining the destination address only d) None of the above

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12.2 Source Routing Theory

2. In source routing, each ring number is identified by a) a 2-digit hexadecimal number (00-FF) b) a 2 digit decimal number (00-99) c) a 3 digit hexadecimal number (000-FFF) d) a 3 digit decimal number (000-999) N.B. A Ring number of 0 is valid, a bridge number of 0 designates the null bridge i.e. the value for terminating a RIF.

3. The control header in the RIF is: a) always exactly 2 bytes long b) at least 2 bytes long c) optional

4. Two source routing bridges with the same bridge number must not a) exist on the same network b) attach to the same ring c) join the same two rings

12.3 Explorer frames

1. If source route bridging is implemented, bridges joined in parallel can provide: a) alternative routes for traffic b) load balancing c) contingency in case of failure d) all of the above

2. If transparent bridging is implemented, bridges joined in parallel can provide: a) alternative routes for traffic b) load balancing c) contingency in case of failure d) all of the above

3. Specifically routed frames are copied by every source routing bridge. a) True b) False

4. Spanning Tree Explorer frames are copied by every source routing bridge. a) True b) False 5. All Routes Explorer frames are copied by every source routing bridge. a) True b) False

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7. The Spanning Tree Protocol is recognised a) by bridges only b) end stations only c) bridges and end stations

12.4 Spanning Tree

1. The Root bridge is the bridge with the a) highest Bridge Id b) highest Bridge Number c) lowest Bridge Id d) lowest Bridge Number

2. The main way to force a bridge to be designated a) lower the Root Path Cost b) lower the Path Cost c) lower the Bridge ID d) lower the Bridge Priority

13. Novell® source route server end station software: Update

The excerpt below comes from the MPR 3 on-line documentation on the MPR 3 Manual CD. This reiterates the care needed when determining an exploration strategy. It also explains some of the logic implemented in the MPR end station software for choosing the most suitable route from those received in the AREs.

By default, the Novell® source route server end station software responds to a Single Route Explorer Frame with an All Routes Explorer Frame. Although you can configure your network to respond to an explorer frame with a Specifically Routed Frame, Novell strongly recommends that you use the default settings.

NOTE: Prior to the 1994 release of the source route server end station software, the behaviour was different. In these earlier releases, the source route server software responded with a Specifically Routed Explorer Frame and could also be configured to respond to an explorer frame with an All Routes Explorer Frame.

Source route end stations specify that when the source receives responses from the destination, it uses one of three criteria to choose the best route:

Which frame is received first (the fastest route)

Which route has the least number of hops (the shortest route)

Which route uses the largest frame size (the widest route)

The ROUTE.COM file chooses the best route based on which frame is received first.

If the DEF (default) option is used, ROUTE.NLM responds in one of two ways:

If the route between two nodes has already been determined and stored in the destination's routing table, resetting the source node causes it to issue a new Single

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Route Explorer Frame to re-establish the route, and the destination responds with an All Routes Explorer Frame (because the route is already known).

If the route is not in the routing table, a new explorer frame from the source causes the destination to respond with a Single Route Explorer Frame (because the route is not already known).

When the destination responds with an All Routes Explorer Frame, the frame is propagated as explained earlier, and the (original) source receives one explorer frame for each route. Because each bridge encountered by the explorer frame records the bridge number and next ring number in the explorer frame's information field, each frame contains its own routing information. The fastest route is chosen.

14. Explorer Framelogs for Windows95

This is an edited log of a Windows95 station starting a session with a Novell server using the pre 1994 ROUTE.NLM which responds incorrectly to the station’s request to perform STE out and ARE back.

The Windows95 station and Framelogger are on the same ring as the root bridge, hence the Root Path Cost of 00000000 held in the SPT frame (BPDU)

--- start of trace logged at 15:19:45.555 Tue Apr 01 1997 --- start 0000F613010A BRIDGE SPT 00 Config 80 Acknowlwdge 52 LLC frame 42 42 UI C 0 Root id : 800000006FC88050, Root path cost : 00000000, Root node addr : 0000F613010A, Root bridge label : 8000 Bridge id : 800000006FC88050, Port id : BACC Node addr : 0000F613010A, Bridge label : 8000 Ring num : BAC, Bridge num : C Message age : 0 sec, Max age : 6 sec Hello time : 2 sec, Forward delay : 4 sec

The Windows95 station sends an STE (limited broadcast) as a SAP request for a server. The full control field is C270 which makes the broadcast indicator is 110 i.e. STE out ARE return.

1.2s 0000F6743FBA ALL_NODES IPX type service advert R=02 53 Routing info (Forward, limited broadcast) = LLC frame e0 e0 UI C 0 IPX frame, type: service advert Info Length: 4 Transport control: 00h Destination: Network=00000000h Node=FFFFFFFFFFFFh Socket=0452h Source: Network=00000000h Node=0000F6743FBAh Socket=4006h Nearest request server advertisment Service type: 0004

The server’s response is 06E0 to the station, a specifically routed frame. It sends the frame back with the same RIF with the direction bit flipped. The frame is a SAP response.

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The server has rejected the stations request to receive back an ARE!

1.420 0000F6371EC9 0000F6743FBA IPX type service advert R=06 119 Routing info (Backward) = <06E0 BAC C CCC 0> LLC frame e0 e0 UI C 0 IPX frame, type: service advert Info Length: 66 Transport control: 00h Destination: Network=00000002h Node=0000F6743FBAh Socket=4006h Source: Network=00000002h Node=0000F6371EC9h Socket=0452h Nearest response server advertisment Service type: 0004 Name: TOKEN_LAB...... Address: Network=BADBABE1h Node=000000000001h Socket=0451h

726.5 0000F613010A BRIDGE SPT 00 Config 80 Acknowlwdge 52 LLC frame 42 42 UI C 0 Root id : 800000006FC88050, Root path cost : 00000000, Root node addr : 0000F613010A, Root bridge label : 8000 Bridge id : 800000006FC88050, Port id : BACC Node addr : 0000F613010A, Bridge label : 8000 Ring num : BAC, Bridge num : C Message age : 0 sec, Max age : 6 sec Hello time : 2 sec, Forward delay : 4 sec

2.0s 0000F613010A BRIDGE SPT 00 Config 80 Acknowlwdge 52 LLC frame 42 42 UI C 0 Root id : 800000006FC88050, Root path cost : 00000000, Root node addr : 0000F613010A, Root bridge label : 8000 Bridge id : 800000006FC88050, Port id : BACC Node addr : 0000F613010A, Bridge label : 8000 Ring num : BAC, Bridge num : C Message age : 0 sec, Max age : 6 sec Hello time : 2 sec, Forward delay : 4 sec

1.9s 0000F6743FBA 0000F6743FBA IPX type unknown R=02 49 Routing info (Forward, limited broadcast) = LLC frame e0 e0 UI C 0 IPX frame, type: unknown Info Length: 0 Transport control: 00h Destination: Network=00000000h Node=0000F6743FBAh Socket=0000h Source: Network=00000000h Node=0000F6743FBAh Socket=0000h

??? 0.930 0000F6743FBA ALL_NODES IPX type route R=02 59 Routing info (Forward, limited broadcast) = LLC frame e0 e0 UI C 0 IPX frame, type: route Info Length: 10 Transport control: 00h Destination: Network=00000000h Node=FFFFFFFFFFFFh Socket=0453h Source: Network=00000000h Node=0000F6743FBAh Socket=4008h Route query Network: BADBABE1

3.690 0000F613010A BRIDGE SPT 00 Config 80 Acknowlwdge 52 LLC frame 42 42 UI C 0 Root id : 800000006FC88050, Root path cost : 00000000, Root node addr : 0000F613010A, Root bridge label : 8000 Bridge id : 800000006FC88050, Port id : BACC

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Node addr : 0000F613010A, Bridge label : 8000 Ring num : BAC, Bridge num : C Message age : 0 sec, Max age : 6 sec Hello time : 2 sec, Forward delay : 4 sec

5.420 0000F6371EC9 0000F6743FBA IPX type route R=06 63 Routing info (Backward) = <06E0 BAC C CCC 0> LLC frame e0 e0 UI C 0 IPX frame, type: route Info Length: 10 Transport control: 00h Destination: Network=00000002h Node=0000F6743FBAh Socket=4008h Source: Network=00000002h Node=0000F6371EC9h Socket=0453h Route response/advertisment Network: BADBABE1 Hops 0001 Ticks 0001

0.510 0000F6743FBA 0000F6371EC9 NOVELL :#1 : Check R=02 56 Routing info (Forward, limited broadcast) = LLC frame e0 e0 UI C 0 IPX frame, type: 11h (exp.) Info Length: 7 Transport control: 00h NOVELL frame, type: First message to server. Frame #: 0 Server slot: ffh Application level: ff00h Packet type (00): Check (OK) return from server

0.500 0000F6371EC9 0000F6743FBA NOVELL :SR : Check R=06 61 Routing info (Backward) = <06E0 BAC C CCC 0> LLC frame e0 e0 UI C 0 IPX frame, type: 11h (exp.) Info Length: 8 Transport control: 00h NOVELL frame, type: Usual reply from server. Frame #: 0 Server slot: 1h Application level: 1h Packet type (00): Check (OK) return from server Packet Param: 0h 0.530 0000F6743FBA 0000F6371EC9 NOVELL :SM : Serv[11] R=06 63 3i Routing info (Forward) = <0660 BAC C CCC 0> LLC frame e0 e0 UI C 0 IPX frame, type: 11h (exp.) Info Length: 10 Transport control: 00h NOVELL frame, type: Usual message to server. Frame #: 1 Server slot: 1h Application level: 1h Packet type (17): Server or other object commands Get File Server's Information

15. Appendix- Glossary of terms B Bridge Id The Bridge Id is composed of the lowest numeric value canonical form (bit reversed in the case of token ring) of the node addresses in the bridge with the management provided parameter called the Bridge Label. The Bridge Label has the highest priority and hence can be used to override the node address portion. Bridge Label See Bridge Id. The default value of this parameter (for Madge bridges) is 32K or 8000 in hex.

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E Exploration The strategy requested by the source route bridging end Strategy station to explore the network and find the server or partner station. This is fixed in the first 3 bits of the Control header at the start of the RIF. The basic options are Spanning Tree out or All Routes Explorer out. Sometimes called Route Discovery Algorithm. M Max. Age This field in an STP frame is set by the Root Bridge. It instructs bridges to discard any STP frames where the Message Age is greater than Max Age, and start to create a new topology if their internal timer exceeds this value. Mesh The term mesh is used to describe a network that has many paths to get between a pair of nodes. Message Age The field in STP frames is an overestimation of the time it takes for the message to propagate from the root to this point in the network. When a bridge receives an STP from a neighbouring bridge with the lowest Root Path Cost, it sets an internal timer to the value of the Message Age in the incoming frame, and uses the value of this timer as the Message Age field for any STP frames it might send out. If it does not receive any further STP frames, the Message Age Field will gradually increase, causing other bridges to question the validity of the current spanning tree path. P Path Cost (PC) This term refers to a parameter within a bridge that determines how suitable a bridge is to be included in the spanning tree. Generally slower bridges should be given higher path costs so that they are only used if a faster bridge is unavailable. R Root bridge For each ring, the Spanning Tree Algorithm works out a single path to the root bridge. By doing this, a single path will exist between all rings in the network often crossing the root bridge. The root bridge should be somewhere in the middle of the logical network. The root bridge is determined in the Spanning Tree Protocol by choosing the bridge with the lowest Bridge Id. Root Path Cost This is the sum of the Path Costs of the bridges between a node and the root bridge. The Spanning Tree Protocol uses this to decide which path to use if there is more than one path between the node and the root bridge. By altering the Path Costs on bridges you can alter the relative Root Path Costs and hence manage the topology of the spanning tree. S

Source Routed A frame whose route is pre-defined by the source. The

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Frame or route must have been found using an STE or an ARE in Specifically the past. If the route is based on a previous ARE, it will Routed Frame probably be designed to take the shortest route, which may include bridges which are not part of the spanning tree. Spanning Tree The term Spanning Tree is used to refer to a network that has only one path to get between any pair of nodes. Spanning Tree The mathematically determined process which runs Algorithm or across source route bridges to set up the spanning tree. Protocol Spanning Tree A frame that tasks a single path to each node in the Explorer (STE) network, following the spanning tree. Only one copy Frame arrives at the destination node. If there is a mesh, the STE frame will not cross paths that are not enabled by the spanning tree. Store and A typical token ring bridge is a Store and Forward device; each Forward device frame to be processed is stored in memory before a decision to forward is made.

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