Huawei Quidway S6500 Series Operation Manual page 400

Operation Manual - STP
Quidway S6500 Series Ethernet Switches
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}.
By comparison, CP1 configuration BPDU is elected as the optimum one. The CP1 is
thus specified as the root port with no modifications made on its configuration BPDU.
However, CP2 will be blocked and its BPDU also remains same, but it will not receive
the data (excluding the STP packet) forwarded from Switch B until spanning tree
calculation is launched again by some new events. For example, the link from Switch B
to 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 will not be updated and retain {0, 10, 0, AP2}.
By comparison, {0, 9, 1, BP2}, the configuration BPDU of CP2, is elected as the
optimum one, CP2 is elected as the root port, whose BPDU will not change, while CP1
will be blocked and retain its BPDU, but it 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 C as down.
Thus the spanning tree is stabilized. The tree with the root bridge A is illustrated in the
Figure 1-5 below.
Switch B
with priority 1
Figure 1-5 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 MAC address. In the updating process of a configuration BPDU, other
configuration BPDUs besides the first four items will make modifications according to
certain rules. The basic calculation process is described below:
Configuration BPDU forwarding mechanism in STP:
Switch A
with priority 0
AP1
5
BP1
BP2
4
CP2
Switch C
with priority 2
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1-8
Chapter 1 MSTP Region-configuration