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 send out the BPDUs and TCN BPDUs.
This triggers a new spanning tree calculation process to establish a new path to restore the
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 calculation involves three important timing parameters: 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 ensure 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
ensure 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.
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 with 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 with 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.