Operation Manual - STP
Quidway S3000-EI Series Ethernet Switches
Chapter 1 MSTP Region-configuration
Huawei Technologies Proprietary
1-8
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, 0, 0, AP2}.
By comparison, 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 Switch A is illustrated in the
Figure 1-5 below.
Switch A
with priority 0
Switch C
with priority 2
Switch B
with priority 1
CP2
BP2
BP1
AP1
4
5
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 switch 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:
z
Configuration BPDU forwarding mechanism in STP:
