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Bridging Principles

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Bridging Principles Bridging Principles 1 By the end of this session you will be able to... n Define bridging modes – Source Routing – Transparent – Source Route Transparent (SRT) n Describe how Spanning Tree functions Token Ring Bridging 2 2 Flexible Frame Forwarding Choice of Techniques Source Route Source Route Transparent Transparent Bridging Bridging Bridging n Transparent u Ethernet and Token Ring u simple to implement u not easy to manage in a complex network n Source Routing u Token Ring u requires management effort to implement u trouble shooting is simplified n SRT u short term combination solution Token Ring Bridging 3 Bridging Techniques Transparent Can be used on both Token Ring and Ethernet networks Nothing is identified so implementation is simple Nothing is identified so locating problems can be difficult on complex networks Source Route Bridging Designed for Token Ring networks Requires each ring and bridge to be identified Locating potential and actual trouble spots is simplified SRT Useful when combing transparent and source routing networks, e.g. when adding a department using ‘the other method’ to a company network. Allows bridges/switches to forward both source routed and transparent frames appropriately. Also allows the bridges/switches to communicate with each other. A short term solution, ultimately MAKE UP YOUR MIND; use source routing OR transparent for the whole network. 3 What is the purpose of a Bridge ? 2 4 1 Ring A Ring B 6 3 5 n Connects two physical rings n Forwards or Filters Frames n Single logical network n Keeps local traffic local Token Ring Bridging 4 Bridges are used to physically connect two rings. It is invisible to the workstations on the rings so, in effect, it makes a single logical ring. Its job is to inspect each frame arriving on either ring and to decide whether it needs to be forwarded to the other or not (i.e. remain on it home ring). This will depend on where the destination address in the frame is. The bridge not only keeps local data traffic local, but MAC management is kept local too. 4 Transparent Bridge Operation 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 1 2 1 2 C D F n Each bridge builds a table containing u Destination addresses it knows u Port on which that address can be found Token Ring Bridging 5 Transparent Bridging A Transparent bridge takes a note of every SOURCE address in each frame that arrives and stores it in a table against the port number at which it arrived. Over a period of time it LEARNS all the MAC addresses on each segment (if only from the AMP and SMPs). Thus if station A above is sending a frame to station F, the first bridge ‘knows’ that F is on its port 2. Now we know that F is on the third ring which is over another bridge, but as far as the first bridge is concerned frames with a source address of F have always arrived at port 2. The first bridge knows nothing of what is happening on the other side of the second bridge. 5 Source Route Bridge Operation B HDR DA SA RIF LLC DATA Trailer E A Ring 00A Ring 00B Ring 00C 1 2 1 2 1 2 C D F n Ring and bridge numbers stored in RIF u Routing Information Field n Routing Information held in Frame u not in Bridge Table Token Ring Bridging 6 Source Route Bridging In this form of bridging the bridge doesn’t need to store anything. Each ring is numbered (3 digits of hex) and each bridge has a 1 digit hex id. As the frame crosses each bridge, the Routing Information Field (RIF) is built up inside the frame. 6 Source Route Bridge Operation RIF B HDR DA SA Ring/bridge pairs LLC DATA Trailer E A Ring 00A Ring 00B Ring 00C 1 2 1 2 1 2 Station C D Server F n RIF built up as explorer frame is broadcast across the rings n Frame reaches server with complete RIF n Server uses RIF to get back to station Token Ring Bridging 7 Workstations use explorer frames to find the address of an intended destination and as the frame crosses the network the RIF is incremented at each bridge. When the frame arrives at its destination the complete path will be stored in the RIF. The destination station can use this to route the reply. The workstation sends out a basic source routing frame. This has the start of the routing information field (RIF) but no actual routing information (since the workstation knows none!) When the frame arrives at the first bridge/switch the lack of routing information tells the bridge that it is the first bridge to see this frame. It puts in the originating ring number, its own bridge number and the ring to which it is forwarding the frame. On a bridge this, of course, can only be one ring, but on a switch it could be one of many. A broadcast (explorer) frame will be forwarded to all output ports in a switch so each will have a different RIF. When the frame reaches the next, and every subsequent, bridge the bridge number and next ring number are added. Thus when the frame arrives at the destination machine it will have a complete path in the RIF. 7 Flagging the frame for Source Routing Token Ring HDR DA SA RIF LLC DATA Trailer Frame Individual/Group bit in the I U Manufacturer ID Serial # Source Address is G L 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 Frame logger will show the true MAC address rather than the bit sequence Token Ring Bridging 8 We need a way of indicating that there is a RIF, i.e. the source station is using Source Routing. This is done by the driver software when the frame is assembled. Although we can send to multiple addresses, we can’t send from multiple addresses, so the way we flag that the frame in a source routing frame is to make the source address a group address - this is obviously never going to happen genuinely. It has the added bonus that should the frame arrive at a transparent bridge, which reads the source address for its routing table, it will be discarded as having an invalid address. 8 Routing Information Field (RIF) HDR DA SA RIF LLC DATA Trailer Maximum of 18 bytes Routing Information Control Field 2 bytes 001 A 002 B 003 0 Ring Ring Ring Bridge Bridge Bridge n 2 bytes minimum u Control information only n 18 bytes maximum: u Control + Ring & Bridge pairs u i.e. maximum hop count (7 bridges ) reached n Last bridge number always 0 u destination node is on a ring Token Ring Bridging 9 The Routing Information Field The RIF consists of two bytes of control information (more later) and a number of ring-bridge combinations. If we are using the IBM definition of source routing this allows only 7 ‘hops’ i.e. allows a frame to cross 7 bridges. This means this part can be 7 ring-bridge combinations = 14 bytes plus the final ring number and a final bridge id of 0 - a further two bytes. Adding in the control byte we have a RIF of 18 bytes. If we are using the IEEE source routing specification, this allows 13 hops, i.e. a RIF of 30 bytes (26 +2 +2). The last bridge id is always 0 since the destination must be on a ring. If we stopped on the ring the RIF would be half a byte short so the 0 is added to complete to a sensible size. Since 0 is not a valid bridge id this could also act as an end of field character if necessary. 9 Constantly Circulating Frames Unique Ring Numbers Control 002 1 004 2 002 0 Ring 004 n Ring number not allowed to be in RIF more than 1 2 3 once n Stops constantly Ring circulating frames 002 Token Ring Bridging 10 If we have more than one bridge between two rings, each bridge must be numbered differently so that the ring number-bridge number is a unique combination. The ring number is also only allowed to be in the RIF once. When the frame from ring 002 crosses bridge 1 and gets to ring 004 the RIF contains: [control] 002 1 004 0 Bridge 2 (or bridge 3) will not allow the frame back on to ring 002 because that ring number is already in the RIF. This check prevents endlessly circulating frames. 10 Source Route Bridging All Routes Explorer (ARE) Frames ARE 2 2 001 3 002 3 003 A B 101 1 102 1 103 Token Ring Bridging 11 There are two types of explorer frames stations can use to find a destination. The first of these is the All Routes Explorer which is exactly what it does. It will be propagated (copied) to all output ports of all bridges/switches so that copies will arrive at the destination having covered all possible routes. In the above example the single ARE will become 4 by the time it reaches the server on ring 3 using the top route and a further one frame will get through the bottom route - one starts, five finish.
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