H3C S9500 Series Operation Manual page 113

Operation Manual – MSTP
H3C S9500 Series Routing Switches
After the comparison processes described in the table above, a spanning tree with
Switch A as the root bridge is stabilized, as shown in
Figure 1-3 A spanning tree with Switch A as the root bridge
Note:
To facilitate description, the spanning tree computing 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 is inferior to the
configuration BPDU of the local port, the port will immediately send out its superior
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 will generate a configuration BPDU with itself as
the root and send out the BPDU. This triggers a new spanning tree computing
process so that a new path is established to restore the network connectivity.
Figure
1-10
Chapter 1 MSTP Configuration
1-3.