Rapid Spanning Tree

Rapid Spanning Tree (RSTP) optimizes convergence in a properly configured network by significantly reducing the time to reconfigure the network‘s active topology when physical topology or configuration parameter changes occur. RSTP is defined in the IEEE 802.1w standard. Spanning Tree‘s primary goal is to ensure a fully connected, loop‐free topology. A secondary goal, realized with the introduction of RSTP, is to move root and designated ports to the forwarding state as quickly as possible.

In a stable topology all the root and designated ports will be forwarding and the alternate and backup ports will be blocking. When there is a network topology change, Spanning Tree recalculates port roles. Ports which are no longer part of the active topology will be put into blocking state. New designated ports will only forward after receiving an acknowledgment or, in the case of a port being connected to a non-RSTP device (802.1d), after a sufficient amount of time has passed.

When a topology change occurs, a change in port operational status or new information contained in BPDUs is immediately acted upon. A new root port moves to forwarding state as soon as any recent former root port is put into blocking state. A designated port moves to forwarding state once the connected device acknowledges agreement with the new topology information. This is typically an exchange of two BPDUs. These rules ensure an orderly transition from the old topology to the new topology by preventing transient loops.