Operation Manual - STP
Quidway S3000 Series Ethernet Switches Chapter 1 RSTP Configuration
1-5
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. 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, 0, 0, BP1}. BP2 updates root
ID with that in the optimum configuration BPDU, the path cost to root with 5, sets the
designated switch 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. {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}.
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.
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