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3Com Switch 8800 Configuration Guide Chapter 38 MSTP Region-configuration
38-8
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 the
message and keep the local BPDU unchanged. When the port receives a
higher-priority configuration BPDU, the switch uses the content in the received
configuration BPDU to change the content of the local BPDU of this port. Then the
switch compare the configuration BPDU of this port to those of other ports on it to elect
the optimum configuration BPDU.
The comparison rules are:
z The configuration BPDU with a smaller root ID has a higher priority.
z If the root IDs are the same, perform the comparison based on root path costs.
The cost comparison is as follows: the path cost to the root recorded in the
configuration BPDU plus the corresponding path cost of the local port is set as S,
the configuration BPDU with a smaller S has a higher priority.
z If the costs of path to the root are also the same, compare in sequence the
designated bridge ID, designated port ID and the ID of the port via which the
configuration BPDU was received.
For the convenience of expression, this example supposes that the optimum
configuration BPDU can be elected just by the comparison of root IDs.
3) Determine the root and designated ports, and update the configuration BPDU of
designated ports.
The port receiving the optimum configuration BPDU is designated to be the root port,
whose configuration BPDU remains unchanged. Switch calculates a designated port