3Com Switch 8800 Advanced Software V5 Configuration Guide

MSTP Overview 99
Tab le 12 Comparison process and result on each device
Device Comparison process
BPDU of the port after
comparison
Switch A ■ Port AP1 receives a configuration BPDU
from Switch B (that is, {1, 0, 1, BP1}).
As the configuration BPDU of the local
port (that is, {0, 0, 0, AP1}) is superior
to the received configuration BPDU,
the received configuration BPDU is
discarded.
■ Port AP2 receives a configuration BPDU
from Switch C (that is, {2, 0, 2, CP1}).
As the BPDU of the local port (that is,
{0, 0, 0, AP2}) is superior to the
received configuration BPDU, the
received configuration BPDU is
discarded.
■ Switch A finds that both the root
bridge and designated bridge in the
configuration BPDUs of all its ports are
Switch A itself, so it assumes itself to
be the root bridge. In this case, it does
not make any change to the
configuration BPDU of each port, and
starts sending out configuration BPDUs
periodically.
AP1: {0, 0, 0, AP1}
AP2: {0, 0, 0, AP2}
Switch B ■ Port BP1 receives a configuration BPDU
from Switch A (that is, {0, 0, 0, AP1}).
As the received configuration BPDU is
superior to that of the local port (that
is, {1, 0, 1, BP1}), Switch B uses the
received configuration BPDU as the
configuration BPDU of BP1.
■ Port BP2 receives a configuration BPDU
from Switch C (that is, {2, 0, 2, CP2}).
As the configuration BPDU of the local
port (that is, {1, 0, 1, BP2}) is superior
to the received configuration BPDU,
Switch B discards the received
configuration BPDU.
BP1: {0, 0, 0, AP1}
BP2: {1, 0, 1, BP2}
■ Switch B compares the configuration
BPDUs of all its ports, and determines
that the configuration BPDU of BP1 is
the optimum one. So, BP1 acts as the
root port, the configuration BPDUs of
which remains unchanged.
■ Based on the configuration BPDU of
BP1 and the path cost of the root port
(5), Switch B generates a designated
port configuration BPDU for BP2 (that
is, {0, 5, 1, BP2}).
■ Switch B compares the generated
configuration BPDU (that is, {0, 5, 1,
BP2}) with the configuration BPDU of
BP2. As the former is superior, BP2 acts
as a designated port, and Switch B
sends the generated configuration
BPDU through BP2 periodically.
Root port BP1:
{0, 0, 0, AP1}
Designated port BP2:
{0, 5, 1, BP2}