3Com Switch 8800 Family Configuration Guide

128 CHAPTER 17: MSTP REGION-CONFIGURATION

Figure 29 Ethernet switch networking

To facilitate the descriptions, only the first four parts of the configuration BPDU are

described in the example. They are root ID (expressed as Ethernet switch priority),

path cost to the root, designated bridge ID (expressed as Ethernet switch priority)

and the designated port ID (expressed as the port number). As illustrated

Figure 29, the priorities of Switch A, B and C are 0, 1 and 2 and the path costs of

their links are 5, 10 and 4 respectively.

1 Initial state

When initialized, each port of the switches generates the configuration BPDU

taking itself as the root with a root path cost as 0, designated bridge IDs as their

own switch IDs and the designated ports as their ports.

Switch A:

Configuration BPDU of AP1: {0, 0, 0, AP1}

Configuration BPDU of AP2: {0, 0, 0, AP2}

Switch B:

Configuration BPDU of BP1: {1, 0, 1, BP1}

Configuration BPDU of BP2: {1, 0, 1, BP2}

Switch C:

Configuration BPDU of CP2: {2, 0, 2, CP2}

Configuration BPDU of CP1: {2, 0, 2, CP1}

2 Select the optimum configuration BPDU

Every switch transmits its configuration BPDU to others. When a port receives a

configuration BPDU with a lower priority than that of its own, the switch discards

Switch A

with priority 0

Switch C

with priority 2

Switch B

with priority 1

CP2

BP2

CP1

BP1

AP2

AP1