3Com WX3000 operation manual 3The final calculated spanning tree

Figure 1-3The final calculated spanning tree

Device A

With priority 0

AP 1	AP 2
5
BP 1
BP 2	4
Device B	CP 2
With priority 1

Device C

With priority 2

To facilitate description, the spanning tree calculation process in this example is simplified, while the actual process is more complicated.

2)The BPDU forwarding mechanism in STP

zUpon 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 interval of hello time.

zIf it is the root port that received the configuration BPDU and the received configuration BPDU is superior to the configuration BPDU of the port, the device will increase message age carried in the configuration BPDU by a certain rule and start a timer to time the configuration BPDU while it sends out this configuration BPDU through the designated port.

zIf the configuration BPDU received on the designated port has a lower priority than the configuration BPDU of the local port, the port will immediately sends out its better configuration BPDU in response.

zIf 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. In this case, the device generates configuration BPDUs with itself as the root bridge and sends configuration BPDUs and TCN BPDUs. This triggers a new spanning tree calculation so that a new path is established to restore the network connectivity.

However, the newly calculated configuration BPDU will not be propagated throughout the network immediately, so the old root ports and designated ports that have not detected the topology change continue forwarding data through the old path. If the new root port and designated port begin to forward data as soon as they are elected, a temporary loop may occur.

3)STP timers

The following three time parameters are important for STP calculation:

zForward delay, the period the device waits before state transition.

A link failure triggers a new round of spanning tree calculation and results in changes of the spanning tree. However, as new configuration BPDUs cannot be propagated throughout the network immediately, if the new root port and designated port begin to forward data as soon as they are elected, loops may temporarily occur.

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