3Com 5500-SI Configuration Manual page 541

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, it will discard the
message and keep the local BPDU unchanged. When a higher-priority configuration
BPDU is received, the local BPDU is updated. And the optimum configuration BPDU
will be elected through comparing the configuration BPDUs of all the ports.
The comparison rules are:
The configuration BPDU with a smaller root ID has a higher priority
■
If the root IDs are the same, perform the comparison based on root path costs.
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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 X,
the configuration BPDU with a lower X has a higher priority.
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 using which the
configuration BPDU was received.
3 Specify the root port and designated port, block the redundancy link and update the
configuration BPDU of the designated port.
The port receiving the optimum configuration BPDU is designated to be the root port,
whose configuration BPDU remains the same. The Switch calculates a designated port
BPDU for every other port: substituting the root ID with the root ID in the
configuration BPDU of the root port, the cost of path to root with the value made by
the root path cost plus the path cost corresponding to the root port, the designated
bridge ID with the local Switch ID and the designated port ID with the local port ID.
The Switch compares the calculated BPDU with the BPDU of the corresponding port.
If the BPDU of the corresponding port is better, the BPDU of the port remains the
same. If the calculated BPDU is better, the port will be the designated port, and the
port BPDU will be modified by the calculated BPDU.
The comparison process of each Switch is as follows.
Switch A:
■
AP1 receives the configuration BPDU from Switch B and finds out that the local
configuration BPDU priority is higher than that of the received one, so it discards
the received configuration BPDU. The configuration BPDU is processed on the AP2
in a similar way. Thus Switch A finds itself the root and designated bridge in the
configuration BPDU of every port; it regards itself as the root, retains the
configuration BPDU of each port and transmits configuration BPDU to others
regularly thereafter. By now, the configuration BPDUs of the two ports are as
follows:
Configuration BPDU of AP1: {0, 0, 0, AP1}.
Configuration BPDU of AP2: {0, 0, 0, AP2}.
Switch B:
■
BP1 receives the configuration BPDU from Switch A and finds that the received
BPDU has a higher priority than the local one, so it updates its configuration BPDU.
BP2 receives the configuration BPDU from Switch C and finds that the local BPDU
priority is higher than that of the received one, so it discards the received BPDU.
By now the configuration BPDUs of each port are as follows: Configuration BPDU
of BP1: {0, 0, 0, AP1}, Configuration BPDU of BP2: {1, 0, 1, BP2}.
STP Overview 541