802.1W Rapid Spanning Tree (RSTP)

Earlier implementation by RUCKUS of Rapid Spanning Tree Protocol (RSTP), which was 802.1W Draft 3, provided only a subset of the IEEE 802.1W standard; whereas the 802.1W RSTP feature provides the full standard. The implementation of the 802.1W Draft 3 is referred to as RSTP Draft 3.

RSTP Draft3 will continue to be supported on RUCKUS devices for backward compatibility. However, customers who are currently using RSTP Draft 3 should migrate to 802.1W.

The 802.1W feature provides rapid traffic reconvergence for point-to-point links within a few milliseconds (0 - 500 milliseconds), following the failure of a bridge or bridge port. This reconvergence occurs more rapidly than the reconvergence provided by the 802.1D Spanning Tree Protocol (STP)) or by RSTP Draft 3.

Note: This rapid convergence will not occur on ports connected to shared media devices, such as hubs. To take advantage of the rapid convergence provided by 802.1W, make sure to explicitly configure all point-to-point links in a topology.

The convergence provided by the standard 802.1W protocol occurs more rapidly than the convergence provided by previous spanning tree protocols because of the following:

  • Classic or legacy 802.1D STP protocol requires a newly selected Root port to go through listening and learning stages before traffic convergence can be achieved. The 802.1D traffic convergence time is calculated using the following formula.

2 x FORWARD_DELAY + BRIDGE_MAX_AGE.

If default values are used in the parameter configuration, convergence can take up to 50 seconds. (In this document STP will be referred to as 802.1D.)

  • RSTP Draft 3 works only on bridges that have Alternate ports, which are the precalculated "next best root port". (Alternate ports provide back up paths to the root bridge.) Although convergence occurs from 0 - 500 milliseconds in RSTP Draft 3, the spanning tree topology reverts to the 802.1D convergence if an Alternate port is not found.
  • Convergence in 802.1w bridge is not based on any timer values. Rather, it is based on the explicit handshakes between Designated ports and their connected Root ports to achieve convergence in less than 500 milliseconds.