3Com 8807 Configuration Manual page 129

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:
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.
■
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.
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 BPDU for every 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.
Switch compares the calculated BPDU with the BPDU of corresponding port. If the
BPDU of corresponding port is better, the port is blocked, and the BPDU of the
port remains unchanged. The port will not forward data and only receive but not
send BPDU. If the calculated BPDU is better, the port will be the designated port,
and the port BPDU will be modified by the calculated BPDU and sent out regularly.
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}.
Introduction to MSTP 129