3Com 4210 PWR Configuration Manual page 148

146 C 14: MSTP C
HAPTER
n
ONFIGURATION
Figure 48 The final calculated spanning tree
Device A
With priority 0
AP 1
5
BP 1
BP 2
Device B
With priority 1
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 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:
Forward delay, the period a 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
AP 2
4
CP 2
Device C
With priority 2