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.