3Com 8800 Configuration Manual page 545

3Com Switch 8800 Configuration Guide Chapter 38 MSTP Region-configuration
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. For
example, the link from Switch B to Switch C is down or the port receives any better
configuration BPDU
Thus, the spanning tree is stabilized. The tree with the root bridge A is illustrated in the
Figure 38-8.
Switch A
AP1
priority level as 0
5
BP1
BP2
4
Switch B
CP2
priority level as 1
Switch C
priority level as 2
Figure 38-8 The final stabilized spanning tree
To facilitate the descriptions, the description of the example is simplified. For example,
the root ID and the Designated bridge ID in actual calculation should comprise both
switch priority and switch MAC address. Designated port ID should comprise port
priority and port ID. In the updating process of a configuration BPDU, other
38-10