MSTP Overview 205
Figure 55 The final calculated spanning tree
n
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 send
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 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.
3 STP timers
STP calculations need three important timing parameters: forward delay, hello
time, and max age.
■ Forward delay is the delay time for device state transition. A path failure will
cause re-calculation of the spanning tree, and the spanning tree structure will
change accordingly. However, the new configuration BPDU as the calculation
result cannot be propagated throughout the network immediately. If the newly
elected root port and designated ports start to forward data right away, a
temporary loop is likely to occur. For this reason, as a mechanism for state
Device A
With priority 0
Device B
With priority 1
Device C
With priority 2
AP 1
BP 1
BP 2
CP 2
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