Implementing the Rapid Spanning Tree Protocol – RSTP
The Rapid Spanning Tree Protocol – RSTP

RSTP is designed to significantly speed the recalculation of the Spanning Tree when a Layer 2 network topology changes. It is an IEEE standard that incorporates many of the concepts used in the Cisco-proprietary STP enhancements, but redefines the base operation of the STP port roles and states, and the BPDUs.

RSTP is proactive instead of passive and has negated the need for the 802.1D delay timers. RSTP (802.1w) supersedes 802.1D, while still remaining backward compatible. The 802.1D terminology remains primarily the same and most parameters remain unchanged. In addition, 802.1w is capable of reverting back to 802.1D to interoperate with legacy switches on a per-port basis. However, in doing so, the benefits of 802.1w over 802.1D are negated.

In a switched domain, there can be only one forwarding path toward a single reference point; this is the root bridge. The RSTP spanning-tree algorithm (STA) elects a root bridge in exactly the same way as 802.1D elects a root.

However, there are critical differences that make RSTP the preferred protocol for preventing Layer 2 loops in a switched network environment. Many of the differences stem from the Cisco proprietary enhancements. The Cisco-based RSTP enhancements have these characteristics:

  • They are integrated into the protocol at a low level.
  • They are transparent.
  • They require no additional configuration.
  • They generally perform better than the Cisco-proprietary 802.1D enhancements.
  • BPDU carries information about port roles and is sent to neighbor switches only.

Because the RSTP and the Cisco-proprietary enhancements are functionally similar, features such as UplinkFast and BackboneFast are not compatible with RSTP.