H3C Technologies H3C SecPath F1000-E - Manual (Page 88)

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• 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 and send out the 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 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 may occur.
STP timers
STP calculation involves the following timers: forward delay, hello time, and max age.
• Forward delay is 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 that the new configuration BPDU has propagated throughout the network.
• Hello time is the time interval at which a device sends hello packets to the surrounding devices to
make sure that the paths are fault-free.
• Max age is 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.
Introduction to RSTP
Developed based on the 802.1w standard of IEEE, RSTP is an optimized version of STP. It achieves rapid
network convergence by allowing a newly elected root port or designated port to enter the forwarding
state much quicker under certain conditions than in STP.
NOTE:
•
In RSTP, a newly elected root port can enter the forwarding state rapidly if this condition is met: the old
root port on the device has stopped forwarding data and the upstream designated port has started
forwarding data.
• In RSTP, a newly elected designated port can enter the forwarding state rapidly if this condition is met:
the designated port is an edge port or a port connected to a point-to-point link. If the designated port
is an edge port, it can enter the forwarding state directly. If the designated port is connected to a
point-to-point link, it can enter the forwarding state immediately after the device undergoes handshake
with the downstream device and gets a response.