3Com 8807 Configuration Manual page 130

130 C 17: MSTP R -
HAPTER EGION CONFIGURATION
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}.
Switch B compares the configuration BPDUs of the ports and selects the BP1 BPDU
as the optimum one because the current configuration BPDU {0, 5, 0, AP1} of BP1
has a higher priority than the configuration BPDU {1, 0, 1, BP2} of BP2. Thus BP1 is
elected as the root port and the configuration BPDUs of Switch B ports are
updated as follows.
The configuration BPDU of the root port BP1 retains as {0, 5, 0, AP1}. BP2 updates
root ID with that in the optimum configuration BPDU, the path cost to root with 5,
sets the designated bridge as the local switch ID and the designated port ID as the
local port ID. Thus, the configuration BPDU becomes {0, 5, 1, BP2}.
Then, all the designated ports of Switch B transmit the configuration BPDUs
regularly.
Switch C:
CP2 receives from the BP2 of Switch B the configuration BPDU {1, 0, 1, BP2} that
has not been updated and then the updating process is launched. The
configuration BPDU is updated as {1, 0, 1, BP2}.
CP1 receives the configuration BPDU {0, 0, 0, AP2} from Switch A and Switch C
launches the updating. The configuration BPDU is updated as {0, 0, 0, AP2}.
Now, the configuration BPDU of CP1 is {0, 10, 0, AP2}, which has a higher priority
than that of CP2.By comparison, CP1 configuration BPDU is elected as the
optimum one. The CP1 is thus specified as the root port without modifying its
configuration BPDU. However, CP2 will be blocked and its BPDU also remains
unchanged, but it will not receive the data (excluding the STP packets) forwarded
from Switch B until spanning tree calculation is launched again by some new
events. For example, the link from Switch B to Switch C is down or the port
receives any better configuration BPDU.
CP2 will receive the updated configuration BPDU, {0, 5, 1, BP2}, from Switch B.
Since this configuration BPDU is better then the old one, the old BPDU will be
updated to {0, 5, 1, BP2}.
Meanwhile, CP1 receives the configuration BPDU from Switch A but its
configuration BPDU is not updated and retain {0, 10, 0, AP2}.
By comparison, {0, 9, 1, BP2}, the configuration BPDU of CP2, is elected as the
optimum one. Thus, CP2 is elected as the root port, whose BPDU will not change,
while CP1 is blocked, its BPDU is retained, and will not receive the data forwarded
from Switch A until spanning tree calculation is triggered again by some changes.