292
Figure 314 The final calculated spanning tree
STP configuration BPDU forwarding mechanism
• Upon network initiation, every device regards itself as the root bridge, generates configuration
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
superior to the configuration BPDU of the port, the device increases the message age carried in
the configuration BPDU following a certain rule and starts a timer to time the configuration
BPDU while sending this configuration BPDU out of the designated port.
• If the configuration BPDU received on a designated port has a lower priority than the
configuration 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 will no longer receive new configuration
BPDUs and the old configuration BPDUs will be discarded due to timeout. The device will
generate configuration BPDUs with itself as the root. This triggers a new spanning tree
calculation process to establish a new path 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 ports and designated ports begin to
forward data as soon as they are elected, a temporary loop might occur.
STP timers
STP calculation involves the following timers:
• Forward delay—The delay time for device 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 the new configuration BPDU has propagated throughout the network.
• Hello time—The time interval at which a device sends hello packets to the surrounding devices
to make sure the paths are fault free.
• Max age—A parameter used to determine whether a configuration BPDU held by the device
has expired. A configuration BPDU beyond the max age will be discarded.
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