Operation Manual – MSTP
H3C S3610&S5510 Series Ethernet Switches
Chapter 1 MSTP Configuration
1-10
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
z
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.
z
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.
z
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 send out its
better configuration BPDU in response.
z
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 sends out the BPDU. This triggers a new spanning tree calculation
process 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 along 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.
