Rapid Spanning Tree Protocol overview

Rapid Spanning Tree Protocol (RSTP) is a way to provide rapid traffic reconvergence for point-to-point links within a few milliseconds (< 500 milliseconds), following the failure of a bridge or bridge port.

The STP (802.1d) standard was designed at a time when recovering connectivity after an outage within a minute or so was considered adequate performance. With the advent of Layer 3 switching in LAN environments, bridging competes with routed solutions where protocols such as OSPF are able to provide an alternate path in less time.

RSTP can be seen as evolution of the STP standard. It provides rapid convergence of connectivity following the failure of bridge, a bridge port, or a LAN. It provides rapid convergence of edge ports, new root ports, and ports connected through point-to-point links. The port, which qualifies for fast convergence, is derived from the duplex mode of a port. A port operating in full-duplex will be assumed to be point-to-point, while a half-duplex port will be considered as a shared port by default. This automatic setting can be overridden by explicit configuration.

RSTP is designed to be compatible and interoperate with STP. However, the benefit of RSTP fast convergence is lost when interacting with legacy STP (802.1d) bridges since RSTP downgrades itself to STP when it detects a connection to a legacy bridge.

The states for every Layer 2 interface running the RSTP are as follows:


State	Action
Learning	The interface prepares to participate in frame forwarding.
Forwarding	The interface forwards frames.
Discarding	The interface discards frames. Ports in the discarding state do not take part in the active topology and do not learn MAC addresses. Note: The STP disabled, blocking, and listening states are merged into the RSTP discarding state.

The RSTP port roles for the interface are also different. RSTP differentiates explicitly between the state of the port and the role it plays in the topology. RSTP uses the root port and designated port roles defined in STP, but splits the blocked port role into backup port and alternate port roles:


Backup port	Provides a backup for the designated port and can only exist where two or more ports of the switch are connected to the same LAN; the LAN where the bridge serves as a designated switch.
Alternate port	Serves as an alternate port for the root port providing a redundant path towards the root bridge.

Only the root port and the designated ports are part of the active topology; the alternate and backup ports do not participate. When the network is stable, the root and designated ports are in the forwarding state, while the alternate and backup ports are in the discarding state. When there is a topology change, the new RSTP port roles allow a faster transition of an alternate port into the forwarding state.

For more information about spanning trees, see the introductory sections in GUID-4C729258-344A-425E-97B9-2E26847EDA56.shtml.