3Com Switch 4800G 24-Port - Page 203

MSTP Overview 203
Device B ■ Port BP1 receives the configuration BPDU of Device A {0, 0,
0, AP1}. Device B finds that the received configuration
BPDU is superior to the configuration BPDU of the local
port {1, 0, 1, BP1}, and updates the configuration BPDU of
BP1.
■ Port BP2 receives the configuration BPDU of Device C {2, 0,
2, CP2}. Device B finds that the configuration BPDU of the
local port {1, 0, 1, BP2} is superior to the received
configuration BPDU, and discards the received
configuration BPDU.
BP1: {0, 0, 0,
AP1}
BP2: {1, 0, 1,
BP2}
■ Device B compares the configuration BPDUs of all its ports,
and determines that the configuration BPDU of BP1 is the
optimum configuration BPDU. Then, it uses BP1 as the root
port, the configuration BPDUs of which will not be
changed.
■ Based on the configuration BPDU of BP1 and the path cost
of the root port (5), Device B calculates a designated port
configuration BPDU for BP2 {0, 5, 1, BP2}.
■ Device B compares the calculated configuration BPDU {0, 5,
1, BP2} with the configuration BPDU of BP2. If the
calculated BPDU is superior, BP2 will act as the designated
port, and the configuration BPDU on this port will be
replaced with the calculated configuration BPDU, which
will be sent out periodically.
Root port BP1:
{0, 0, 0, AP1}
Designated port
BP2:
{0, 5, 1, BP2}
Table 41 Comparison process and result on each device
Device Comparison process
BPDU of port
after
comparison