H3C SR8800 Configuration Manual page 42

Figure 11 Topology of the final calculated spanning tree
NOTE:
The spanning tree calculation process in this example is only simplified process.
The configuration BPDU forwarding mechanism of STP
The configuration BPDUs of STP are forwarded following these guidelines:
Upon network initiation, every router regards itself as the root bridge, generates configuration
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BPDUs with itself as the root, and sends the configuration BPDUs at a regular hello interval.
If it is the root port that received a configuration BPDU and the received configuration BPDU is
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superior to the configuration BPDU of the port, the router increases the message age carried in the
configuration BPDU following a certain rule and starts a timer to time the configuration BPDU while
sending out this configuration BPDU through the designated port.
If the configuration BPDU received on a designated port has a lower priority than the configuration
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BPDU of the local port, the port immediately sends out its own configuration BPDU in response.
• If a path becomes faulty, the root port on this path no longer receives new configuration BPDUs and
the old configuration BPDUs will be discarded due to timeout. The router generates a configuration
BPDU with itself as the root and sends out the BPDUs and TCN BPDUs. This triggers a new spanning
tree calculation process to establish a new path to restore the network connectivity.
However, the newly calculated configuration BPDU cannot 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 ports and designated ports begin to
forward data as soon as they are elected, a temporary loop may occur.
STP timers
STP calculation involves the following timers: forward delay, hello time, and max age.
Forward delay
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Forward delay is the delay time for state transition.
A path failure can cause spanning tree re-calculation to adapt the spanning tree structure to the
change. However, the resulting new configuration BPDU cannot propagate throughout the
network immediately. If the newly elected root ports and designated ports start to forward data
right away, a temporary loop is likely to occur.
For this reason, as a mechanism for state transition in STP, the newly elected root ports or
designated ports require twice the forward delay time before transiting to the forwarding state to
make sure that the new configuration BPDU has propagated throughout the network.
Hello time
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