IEEE 802.1w-Rapid Spanning Tree Protocol

Rapid Spanning Tree Protocol (RSTP) is an evolution of the 802.1D STP standard. RSTP is a Layer 2 loop prevention algorithm like 802.1D; however, RSTP achieves rapid failover and convergence times because RSTP is not a timer-based Spanning Tree Algorithm (STA) like 802.1D, but rather a handshake-based STA. Therefore, RSTP offers an improvement of 30 seconds or more (as compared to 802.1D) in transitioning a link into a Forwarding state.

The only three port states in RSTP are

• Learning
• Forwarding
• Discarding

The Disabled, Blocking, and Listening states from 802.1D have been merged into a unique 802.1w Discarding state, which is a nonforwarding and nonparticipating RSTP port-state.

Rapid transition is the most important feature introduced by 802.1w. The legacy STA passively waited for the network to converge before moving a port into the Forwarding state. Achieving faster convergence was a matter of tuning the conservative default timers, often sacrificing the stability of the network.

RSTP can actively confirm that a port can safely transition to Forwarding without relying on any timer configuration. A feedback mechanism operates between RSTP-compliant bridges. To achieve fast convergence on a port, the RSTP relies on two new variables:

• Edge ports: The edge port concept basically corresponds to the PortFast feature. The idea is that ports that directly connect to end stations cannot create bridging loops in the network and can, thus, directly transition to Forwarding (skipping the 802.1D Listening and Learning states). An edge port does not generate topology changes when its link toggles. Unlike PortFast though, an edge port that receives a BPDU immediately loses its edge port status and becomes a normal spanning-tree port.
• Link type: RSTP can achieve only rapid transition to Forwarding on edge ports and on point-to-point links. The link type is automatically derived from the duplex mode of a port. A port operating in full-duplex will be assumed to be point-to-point, whereas a half-duplex port will be considered as a shared port by default. In today’s switched networks, most links operate in full-duplex mode and are, therefore, treated as point-to-point links by RSTP. This makes them candidates for rapid transition to forwarding.

Like STP, you can enable RSTP globally on a per-VLAN basis, also referred to as Rapid-Per-VLAN-Spanning Tree (Rapid-PVST) mode, using the following command:

Switch(config)# spanning-tree mode rapid-pvst