H3C S3100-52P Operation Manual page 254

Operation Manual – MSTP
H3C S3100-52P Ethernet Switch
Figure 1-3 The final calculated spanning tree
Note:
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
Upon network initiation, every switch 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.
If 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.
If 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.
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. 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
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Chapter 1 MSTP Configuration