LAN Interconnection 2
Repeaters, Bridges, Routers and Gateways
g LAN interconnection refers to the ability to inter- network LANs, MANs, and WANs, through relays.
b A layer n relay is a device that interconnects two systems not directly connected to each other (OSI Ref. Model) ` Layer n relay shares a common layer n with other systems, but does not participate in layer n+1 protocol g Terminology is non uniformly used in the litera- ture
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Interconnecting LANs
g There are four generic devices for LAN intercon- nection
b Repeater : Physical layer relay
b Bridge : Data link layer relay
b Router : Network layer relay
b Gateway : Any relay at higher layer than net- work layer g One final category combines bridges and routers functionalities
b Brouters
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Relays, Bridges, Routers and Gateways
End System End System
Relays
Network Relays
Bridge Repeater
Physical Media
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Shared-Bandwidth LANs
Thick Ethernet Trunk Cable Maximum Trunk Link, 500 Meters Transceiver
Terminating Resistor at each end Transceiver Cable
Bridge
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Dedicated-Bandwidth LANs Structured Wiring
Work Location Wall Plate Wiring Closest LAN Hub
Patch Panel Horizental Distribution V (UTP Cat 5) e r t i c B a u l i l d i n g
UTP B a 100 m distance D c i k VD s b µ t o Fiber (62.5/125 m) r n 2 Km distance i e b u t i o n
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Repeaters
g Repeaters are used to interconnect homogene- ous networks, and to extend their range of opera- tions g Repeaters pass signals in both directions g Their main functions include
b Amplifying signals
b Regenerating signal
b Signals are not changed except for ‘‘cleaning’’ g Repeaters have no effect on protocols
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Standalone Repeaters
g Repeaters forward packets to all ports
b Shared Network g All segments interconnected by means of repeaters are in one Electrical Collision Domain
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Electrical Collision Domain
g All collisions occurring in a single collision domain must be detected by the nodes causing the collisions before they terminate their transmissions
b This has to hold for all network nodes, includ- ing the ones at the farthest ends of the net- work g As a result, the network diameter is directly related to the minimum packet size, which is 512 bits
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Electrical Collision Domain
DTE Delay Cable Delay Repeater Delay Cable Delay DTE Delay
g To handle collision appropriately, the following rule must hold true 2 × ( Total repeater delays ) + 2 × ( Total cable delays ) + 2 × ( Total DTE delays ) < 512 bit times
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Bridges For Shared LANs
g Bridges allow for the simplest and most common form of internetworking, interconnection of homogeneous networks g IEEE 802 committees developed standards for bridges to interconnect different types of LANs b Rather a complex task
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Typical Bridge Configuration
Host Host
LAN Bridge Bridge LAN
Host Host
Bridge-to-Bridge MAC Protocol Protocol
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Why Bridges?
g Several reasons for the use of bridges to connect LANs
b Performance, by building ‘‘firewalls’’,
b Reliability, network can be partitioned into self contained units
b Security, control and monitor traffic
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Bridges Characteristics
g Bridges operate at the data link layer
b Usually the MAC layer of LANs ` The basic bridge, with no special features, does not look any further than the address fields
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Bridges
g Bridges do not interfere with higher level proto- cols
b Transparently pass traffic between upper- layer running different protocols (XNS, TCP/IP, and OSI)
b Only devices running the same protocol can communicate with one another
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Bridges Main Advantages
g Bridges can connect LANs that use different phy- sical media such as coax cable, fiber optics, or twisted pairs. g Bridges can connect LANs that use different media access control protocols media such as CSMA/CD and Token Ring.
b The possible difference in maximum frame size must be handled by upper layers protocol ` Passing through the bridge, the packet does not violate the maximum length on any network segment
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Bridges Basic Functionalities
g A Bridge, between network 1 and 2, performs the following functions
b Reads all frames on network 1, and accepts those addressed to network 2
b Retransmits accepted frames onto network 2, using the MAC protocol for network 2
b Performs equivalent functions on network 2 to network 1 traffic
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Bridges Basic Operations
Host A Host B Layers 4-7
Network Layer PKT PKT Bridge
LLC PKT PKT PKT
MAC 802.3PKT 802.3PKT 802.4PKT 802.4PKT
Physical 802.3PKT 802.3PKT 802.4PKT 802.4PKT
802.3PKT 802.4PKT
CSMA/CD LAN Token Bus LAN
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Bridges Over Point-To-Point Links
Station Station User User ( 1 ) ( 1 ) LLC LLC ( 2) ( 2) Bridge Bridge MAC MAC ( 3) MAC Link Link MAC ( 3) PHY ()3 ()4 (4) ( 3 ) PHY PHY PHY PHY PHY LAN LAN
Packet Data Unit 1 : User Data
Packet Data Unit 2 : LLC-H User Data
Packet Data Unit 3: MAC-H LLC-H User Data MAC-T
Packet Data Unit 4 : Link-HMAC-H LLC-H User Data MAC-T Link-T
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Routing with Bridges
g Most basic form of bridges, ‘‘No Frills’’ bridges perform the following operations :
b Promiscuously, listen to every packet transmitted
b Store each packet until it can be transmitted
b When ready transmit the packet on all adja- cent LANs, except the one on which it was received
b Preferably, operates transparently not to cause protocols on stations to fail
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Basic Bridges
g ‘‘No Frills’’ bridges extend the capabilities of the LANs
b Limits collisions in CSMA/CD
b Extend the maximum number of stations in 802.5
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Basic Bridges
g ‘‘No Frills’’ may waste total LAN bandwidth
b Manual insertion of addresses in the database ` May involve difficult management g If it were for them to know which stations were on which LANs, bridges may become more efficient
b Bridges must be equipped with greater rout- ing capabilities
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Routing with Bridges
g Three basic routing strategies :
b Fixed routing ` Only suitable for small LANs ` Mostly proprietary protocols
b Spanning tree, developed by IEEE 802.1 ` Intended for use in interconnecting LANs with similar and dissimilar MAC Standards, (802.3, 802.4, 802.5, FDDI) ` Routing technique is used is referred to as Spanning Tree
b Source Routing, developed by IEEE 802.5 ` Intended for use in interconnecting Token Rings
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Interconnection of Different Types of LANs
To CSMA/CD Token Bus Token Ring From 802.3 802.4 802.5
802.3 1,4 1,2,4,8
802.4 1,5,8,9,10 9 1,2,3,8,9,10
802.5 1,2,5,6,7,10 1,2,3,6,7 6,7
1 Reformat the frame, compute new checksum
2 Reverse the bit order
3 Copy priority, even if not meaningful
4 Generate fictitious priority
5 Discard priority
6 Drain the ring
7 Fake A and C bits
8 Deal with congestion
9 Deal with temporary token handoff
10 Deal with long frames for receiving LAN
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Bridging with Different LANs
g Two techniques are widely used :
b Encapsulation,
b Translation (Mapping)
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Encapsulation
g The frame of the in-bound LAN is included in the information field of the frame of the out-bound LAN g The receiving bridge performs de-encapsulation
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Encapsulation
g Major limitations :
b No standards for encapsulation are defined
b Requires that both bridges subscribe to the same encapsulation proprietary scheme ` Limits interoperability ` Limits transparency
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Translation Concept
g Bridges map the content of incoming incoming frame into the outbound frame that conforms to the outbound LAN g Translation may require higher processing rate than encapsulation
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Translation
g To achieve translation, bridge internal sublayer performs :
b Frame disassembly at the in-bound LAN
b Altering of certain field of the in-bound LAN frame
b Reconstruction of the out-bound LAN frame g These services are defined within the bridge by a set of primitives and parameters
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Bridge Architecture Service Relationship
Higher Layer Entities (Bridge Protocol Entity, Bridge Management,...)
LLC LLC
MAC Relay Entity Media Access Method Independent MAC Services MAC Services ISS ISS
MAC Entity MAC Entity Media Access Media Access Method Dependent Method Dependent
ISS : Internal Sublayer Service
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MAC Dependent Bridge Action
Destination LAN
Source LAN IEEE 802.3 IEEE 802.4 IEEE 802.5
IEEE 802.3 Calculate CRC Calculate CRC Set default user Set Default user priority priority Use default access Use default access priority priority
IEEE 802.4 Discard frame if Use user priority Discard if long frame too long of inbound frame Calculate CRC Calculate CRC Use user priority Use user priority Discard User Priority or default access of inbound frame priority for access Use user priority or default access priority for access
IEEE 802.5 Discard frame if Discard frame if Use user priority too long too long of inbound frame Calculate CRC Calculate CRC Use user priority Discard User Priority Use user priority or default access of inbound frame priority for access Use user priority or default access priority for access
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IEEE 802 Bridges Spanning Tree Bridges
g Spanning Tree Bridges perform three basic func- tions
b Frame forwarding
b Learning station addresses
b Resolving possible loops in the topology by participating in a spanning tree generation algorithm
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Frame Forwarding
PSI PSI FF
FDB FR FT
PSI : Port State Information FF : Frame Forwarding FR : Frame Reception FT : Frame Transmission FDB : Filtering Data Base
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Frame Forwarding
g Each bridge receives frames from incoming LAN ports and transmits frames to outgoing LAN ports g A forwarding database containing entries for all destinations currently known is maintained
b Database entry = (station address, forward port #)
b Database is originally empty, and initialized by flooding g Bridges perform packet forwarding algorithm, g Bridges perform database maintenance through a learning algorithm
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Forwarding and Learning
Bridge Frame received Forwarding on port X
DA found in No Database?
Yes Forward on ports except X
Yes Outband port = X
No
Forward on outband port
Bridge SA found in No Database? Learning Yes
Update entry Add SA to DB and Timer Add new timer
End
Recommended Timer : 300 seconds
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Learning Bridges
Port 1 Port 2 Port 1 Port 2 B1 B2 Port 3
LAN 1 LAN 2 LAN 3
12 34 56
LAN 4
7 8
Event B1 Actions B1 Database
Station 1 sends a message B1 transmits the message [ 1, Port 1 ] to Station 6 on LAN 3 on Port 2 and Port 3 Station 6 sends a message B1 receives the message [ 1, Port 1 ] to Station 1 on LAN 1 on Port 2 and transmits it on [ 6, Port 2 ] Port 1 Station 1 sends a message B1 transmits the message [ 1, Port 1 ] to Station 6 on LAN 3 on Port 2 [ 6, Port 2 ]
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Looping
g Infinite loops may happen
45 LAN 1
B1 B2 B3
LAN 2
12 3
1- Station 4 transmits to Station 3 3- B1 transmits packet on LAN 2 2- All bridges receive the packet, 4- B2 and B3 receive packet on LAN 2, update their databases and queue note that 4 resides now on LAN 2, packet to be transmitted on LAN 2 update their databases and queue packet to be transmitted on LAN 1
.... This may continue for ever ....
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Loop Elimination
g Design a loop free network
b Loops are not bad since they allow a degree of redundancy g Design an algorithm that prunes the topology into a loop-free subset
b Spanning Tree Algorithm
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Spanning Tree
g The spanning tree is developed automatically by an algorithm resident in each bridge
b No network management is required to configure the network g The tree guarantees only one route between two stations in the LAN
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Spanning Tree Basic Steps
g (a) Bridges elect a single bridge, among all the bridges on all the LANs, to be the ‘‘Root Bridge’’ g (b) Bridges calculate the distance of the shortest path from themselves to the root bridge g (c) For each LAN, briges elect a ‘‘Designated Bridge’’ from among all the bridges residing on that LAN
b Elected bridge is the one closest to the ‘‘Root Bridge’’ g (d) Brigdes select the ‘‘Root Port’’ that gives the best path from themselves to the ‘‘Root Bridge’’ g (e) Briges select ports to be included in the span- ning tree
b Selected ports include ‘‘Root Port’’ and any port on which the bridge has been selected ‘‘Designated Bridge’’
b Data traffic is forwarded only to and from ports selected in the spanning tree
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Spanning Tree Bridge Algorithm
g Each port of a bridge is uniquely identified within the bridge
b A separate unique LAN address (48-bit) g Each bridge is assigned a unique identifier,
b In essence, the MAC address of one of its ports and a priority level g A special group MAC address,
b All bridges on this LAN
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Spanning Tree
g Root Bridge
b Bridge with lowest value of bridge identifier
g Path Cost
b Associated with each port on each bridge is a path cost ` Cost of transmitting a frame onto a LAN through that port
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Spanning Tree
g Root port
b The port used toward the first hop on the minimum cost path to the root bridge g Root path cost
b A root path is the path from a bridge to the route bridge with minimum cost
b The root path cost is the cost of the root path
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Spanning Tree
g Designated bridge
b Bridge that logically connects the LAN to the next LAN closer to the root g Designated port
b Port of the designated bridge that attachs the bridge to the LAN
b All traffic, to and from the LAN, passes through the designated port
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Spanning Tree Algorithm
g Determine the root bridge
b Initially, each bridge assumes it is the root bridge
g Determine the root port on all other bridges
g Determine the designated port on each LAN
b This port will be the port with the minimum root path cost
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General Topology Physical
LAN 2
B3 B4
C=10 C=5 Id:45 Id:57
C=10 C=5 B1
C=10 Id:42 LAN 5
C=10 B5
C=5 Id:83
C=5
LAN 1 B2
C=10
C=5 C=5 Id:97
LAN 3
LAN 4
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Spanning Tree Topology Logical
B1
Id 42 Route Path Cost 0
Port Id : 1 Port Id : 2 Path Cost 10 Path Cost 10 D D
LAN 1 LAN 2
B5 R B4 R Port Id : 1 Port Id : 1 Path Cost 5 Path Cost 5 Id 83 Id 57 Route Path Cost 05 Route Path Cost 05
Port Id : 2 Port Id : 2 Path Cost 5 Path Cost 5 D B2 R B3 R
Id 97 Port Id : 1 LAN 5 Route Path Cost 10 Path Cost 5 Id 45 Route Path Cost 10 Port Id : 1 Port Id : 2 Path Cost 5 Path Cost 5 Port Id : 2 D D Path Cost 10
LAN 3 LAN 4
Prof. A. Bruce McDonald Spring’01