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303

Device Comparison process

BPDU of port after

com

arison

After comparison:

• Because the root path cost of CP2 (9) (root path cost of the

BPDU (5) plus path cost corresponding to CP2 (4)) is smaller

than the root path cost of CP1 (10) (root path cost of the

BPDU (0) + path cost corresponding to CP2 (10)), the BPDU

of CP2 is elected as the optimum BPDU, and CP2 is elected

as the root port, the messages of which will not be changed.

• After comparison between the configuration BPDU of CP1

and the calculated designated port configuration BPDU,

port CP1 is blocked, with the configuration BPDU of the port

unchanged, and the port will not receive data from Device A

until a spanning tree calculation process is triggered by a

new event, for example, the link from Device B to Device C

going down.

Blocked port CP2:

{0, 0, 0, AP2}

Root port CP2:

{0, 5, 1, BP2}

After the comparison processes described in Table 140, a spanning tree with Device A as the root bridge

is established as shown in Figure 313.

Figure 313 The final calculated spanning tree

STP configuration BPDU forwarding mechanism

• Upon network initiation, every device regards itself as the root bridge, generates configuration

BPDUs with itself as the root, and sends the configuration BPDUs at a regular hello interval.

• If it is the root port that received a configuration BPDU and the received configuration BPDU is

superior to the configuration BPDU of the port, the device increases the message age carried in the

configuration BPDU following a certain rule and starts a timer to time the configuration BPDU while

sending this configuration BPDU out of the designated port.

• If the configuration BPDU received on a designated port has a lower priority than the configuration

BPDU of the local port, the port immediately sends out its own configuration BPDU in response.

• If a path becomes faulty, the root port on this path will no longer receive new configuration BPDUs

and the old configuration BPDUs will be discarded due to timeout. The device will generate

configuration BPDUs with itself as the root. This triggers a new spanning tree calculation process to

establish a new path to restore the network connectivity.

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