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STA and Reconfigurations

This document provides an in-depth overview of the bridge protocol data units (BPDUs) used in spanning tree protocols within network standards. It explains the concept of the designated bridge, root port, and blocked ports, highlighting how BPDUs are selected and transmitted by bridges. Key concepts include determining root ID and cost, as well as managing port states (forwarding, designated, and blocked). Through examples, it illustrates how these principles are implemented in practice to maintain efficient network topology and reduce loops in LAN configurations.

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STA and Reconfigurations

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  1. STA and Reconfigurations Network Protocols and Standards Autumn 2004-2005 CS573: Network Protocols and Standards

  2. Designated Bridge • If a bridge receives better BPDU on a port than the one it would transmit, it will no longer transmit BPDUs on that port • As a result, only one bridge called the “designated bridge” transmits configuration BPDUs on each LAN CS573: Network Protocols and Standards

  3. Root ID and Cost: determining • Consider B with bridge ID 18 • Best BPDUs on each of its ports are: <RootID>.<Cost>.<TransmitterID> • Port 1 <12>.<93>.<51> • Port 2 <12>.<85>.<47> • Port 3 <81>.<0>.<81> • Port 4 <15>.<31>.<27> • Root bridge ID is determined as 12 • Least cost is determined as 85+1 = 86 • Port 2 is regarded as the Root port …cont… CS573: Network Protocols and Standards

  4. Root ID and Cost: determining • At next opportunity, B will transmit • <12>.<86>.<18> • Better than the best BPDUs on all ports • Ports 1, 3, and 4 are designated • B considers itself designated for those three segments • B continues transmitting BPDUs on those segments (through respective ports) CS573: Network Protocols and Standards

  5. Spanning Tree Ports • For each bridge, what ports are “included” in the spanning tree? • Root Port • On which best of the best BPDUs is received • Designated Port • Connected to segments on which B is designated • Ports selected in the spanning tree are placed in the forwarding state • B will forward data packets to and from those ports CS573: Network Protocols and Standards

  6. Ports Blocked • Bridge B will block a port if it receives a better BPDU on that port than the one it would transmit (except for the root port) • Bridge B will no longer transmit BPDUs on either root port or blocked ports • Data packets are not forwarded to and received from the blocked ports CS573: Network Protocols and Standards

  7. Port 1 Bridge ID 92 Port 5 Port 4 Port 2 Port 3 41.13.90 81.0.81 41.12.111 41.12.315 41.19.125 Example Root Bridge is selected as 41 Least cost to root is 12+1 = 13  Port 4 is selected as root Now B92 will transmit a BPDU 41.13.92 What does it do with the ports? CS573: Network Protocols and Standards

  8. How the Ports are treated? Remember: B92 will transmit a BPDU 41.13.92 • Port 1: 41.13.92 < 81.0.81  DP • Port 2: 41.13.92 < 41.19.125  DP • Port 3: 41.13.92 > 41.12.315  BP • Port 4: 41.13.92  Root Port • Port 5: 41.13.92 > 41.13.90  BP CS573: Network Protocols and Standards

  9. Example Port 1 Bridge ID 92 Port 5 Port 4 Port 2 Port 3 41.13.90 81.0.81 41.12.111 41.12.315 41.19.125 Blocked Port Designated Bridge on this port Designated Bridge on this port Blocked Port Root Port Bridge B92 becomes designated on Ports 1 and 2 It overwrites messages on those two ports CS573: Network Protocols and Standards

  10. Example Port 1 Bridge ID 92 Port 5 Port 4 Port 2 Port 3 41.13.90 81.0.81 41.12.111 41.12.315 41.19.125 What if B92 were actually B15 with a bridge ID 15? CS573: Network Protocols and Standards

  11. Sample Topology – step 1 DP (1.0.1) 1 DP (1.0.1) DP (4.0.4) DP (5.0.5) DP (7.0.7) 4 5 7 DP (4.0.4) DP (5.0.5) DP (7.0.7) DP (8.0.8) DP (6.0.6) DP (10.0.10) 8 6 10 DP (8.0.8) DP (6.0.6) DP (10.0.10) DP (11.0.11) DP (2.0.2) 11 2 DP (11.0.11) DP (2.0.2) DP (0.0.0) DP: Designated Port 0 DP (0.0.0) CS573: Network Protocols and Standards

  12. Sample Topology – step 2 DP (1.0.1) 1 DP (1.0.1) RP RP RP 4 5 7 DP (1.1.4) DP (1.1.5) DP (1.1.7) RP DP (2.1.6) DP (2.1.10) 8 6 10 DP (4.1.8) RP RP DP (0.1.11) DP (0.1.2) 11 2 RP RP DP (0.0.0) RP: Root Port DP: Designated Port 0 DP (0.0.0) CS573: Network Protocols and Standards

  13. Sample Topology – step 3 DP (1.0.1) 1 DP (1.0.1) RP RP RP 4 5 7 DP (1.1.4) DP (1.1.5) BP RP DP (0.2.6) DP (0.2.10) 8 6 10 DP (1.2.8) RP RP BP DP (0.1.2) 11 2 RP RP RP: Root Port DP: Designated Port BP: Blocked Port DP (0.0.0) 0 DP (0.0.0) CS573: Network Protocols and Standards

  14. Sample Topology – step 4 DP (1.0.1) 1 DP (1.0.1) DP (0.3.4) DP (0.3.5) DP (0.3.7) 4 5 7 RP RP RP RP DP (0.2.6) DP (0.2.10) 8 6 10 DP (0.3.8) RP RP BP DP (0.1.2) 11 2 RP RP RP: Root Port DP: Designated Port BP: Blocked Port DP (0.0.0) 0 DP (0.0.0) CS573: Network Protocols and Standards

  15. Sample Topology – step 5 DP (0.4.1) 1 RP DP (0.3.4) BP BP 4 5 7 RP RP RP RP DP (0.2.6) DP (0.2.10) 8 6 10 DP (0.3.8) RP RP BP DP (0.1.2) 11 2 RP RP RP: Root Port DP: Designated Port BP: Blocked Port DP (0.0.0) 0 DP (0.0.0) CS573: Network Protocols and Standards

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