1-9z If a path becomes faulty, the root port on this path will no longer receive new configuration BPDUsand the old configuration BPDUs will be discarded due to timeout. In this case, the device willgenerate 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 thenetwork connectivity.However, the newly calculated configuration BPDU will not be propagated throughout the networkimmediately, so the old root ports and designated ports that have not detected the topology changecontinue forwarding data along the old path. If the new root ports and designated ports begin to forwarddata as soon as they are elected, a temporary loop may occur.STP timersSTP calculation involves three important timing parameters: forward delay, hello time, and max age.z 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 islikely to occur.For this reason, as a mechanism for state transition in STP, the newly elected root ports or designatedports require twice the forward delay time before transiting to the forwarding state to ensure that the newconfiguration BPDU has propagated throughout the network.z Hello time is the time interval at which a device sends hello packets to the surrounding devices toensure that the paths are fault-free.z Max age is a parameter used to determine whether a configuration BPDU held by the device hasexpired. A configuration BPDU beyond the max age will be discarded.Introduction to RSTPDeveloped based on the 802.1w standard of IEEE, RSTP is an optimized version of STP. It achievesrapid network convergence by allowing a newly elected root port or designated port to enter theforwarding state much quicker under certain conditions than in STP.z In RSTP, a newly elected root port can enter the forwarding state rapidly if this condition is met: theold root port on the device has stopped forwarding data and the upstream designated port hasstarted forwarding data.z In RSTP, a newly elected designated port can enter the forwarding state rapidly if this condition ismet: the designated port is an edge port or a port connected with a point-to-point link. If thedesignated port is an edge port, it can enter the forwarding state directly; if the designated port isconnected with a point-to-point link, it can enter the forwarding state immediately after the deviceundergoes handshake with the downstream device and gets a response.
