802.1w Rapid Spanning Tree Protocol

Rapid Spanning Tree Protocol (RSTP or IEEE 802.1w) improves on 802.1D. The protocol incorporates many new features to speed convergence, including incorporation of the ideas presented by Cisco in its enhancements to 802.1D. Although there are many, many improvements with the new technology, the configuration remains almost identical - and the two technologies can coexist. Full benefits are not realized until all systems are running RSTP, however. RSTP requires full-duplex, point-to-point connections between adjacent switches to achieve fast convergence RSTP defines edge ports as those not participating in STP. Edge ports can be statically configured or will be recognized by the PortFast configuration command.

RSTP port states

RSTP port states are simplified from 802.1D and consist of the following:

■ Discarding
■ Learning
■ Forwarding

Also, the port states are no longer tied directly to port roles. For example, a DP could be Discarding, even though it is destined to transition to the Forwarding state.

RSTP port roles

■ Root port—This port role exists in 802.1D, too, and is the "best" path back to the root bridge; it must exist on all nonroot bridges.
■ Designated port—This port role exists in 802.1D, too, and there must be a DP on all segments in the topology. By default, all ports on the root bridge are DPs.
■ Alternative port—This port role is new to 802.1w. This port is a quickly converging backup port to the current DP on a segment.
■ Backup port—This port role is new to 802.1w. This port is a quickly converging backup to the root port for a system.


RSTP BPDUs

All bridges now send BPDUs every hello time period (2 seconds by default). The BPDUs now act as a keepalive—protocol information is aged if no BPDUs are heard for three consecutive hello times.

RSTP proposal and agreement process/topology change mechanism

Convergence occurs on a link-by-link basis in 802.1w. No longer is there a reliance on timers for convergence as there is in 802.1D. A proposal and agreement process replaces the timer methodology of STP and flows downstream from the root device.
In RSTP, only nonedge ports moving to the Forwarding state cause a topology change (TC). The originator of a TC is now responsible for flooding it through the network.

Implementing RSTP

On most Cisco switches, configuring 802.1s (Multiple Spanning Tree, MST) automatically enables RSTP. Cisco did invent a mode of operation that allows you to use RSTP without the implementation of MST. It is called PVST+ mode. You can enable it on a switch with the following command:

spanning-tree mode rapid-pvst