Rapid Spanning Tree Protocol 578

Rapid Spanning Tree Protocol
In the beginning, all bridges were inherently slow and it was accepted by users and applications
developers alike that convergence would be slow. Cisco engineers have worked to develop solutions
that overcame the basic flaws in STP that became obvious only when switching matured
and took over from legacy bridging. All of the previous enhancements to the STP, such as Port-
Fast, UplinkFast, and BackboneFast, have been proprietary.
Rapid Spanning Tree Protocol (RSTP), which has been standardized as 802.1w, can be
regarded as a replacement for the proprietary extensions. Recalling the two core concepts of the
802.1D STP from Chapter 15, let’s compare the old with the new.
First, 802.1D specifies that there are five different states that a port can be in. Each state is
accompanied by a port mode, so a blocking port, for example, cannot be a root or designated port.
RSTP assumes that three of these states can be regarded as essentially the same from the perspective
of other switches. Listening, blocking, and disabled modes are all characterized by the
facts that they do not forward frames and they do not learn MAC addresses, so RSTP places
them all into a new mode: discarding. Learning and forwarding ports remain more or less the
same. The effect of this change is to decouple the port states from the port roles.
The second big difference is the timing operation. In 802.1D STP, bridges would only send
out a BPDU when they received one on their root port. These legacy bridges essentially act as
forwarding agents for BPDUs that are generated by the root. In contrast, 802.1w-enabled
switches send out BPDUs every hello time, containing current information.
The combination of these two changes forces spanning tree to operate in a much faster mode,
with convergence being achieved in just a few seconds (typically about three times the two-second
update timer), largely because if a switch fails to receive BPDUs on an interface for seconds, it presumes
that the port at the other end of the link is down.
This rapid transition to the forwarding state, caused by switches no longer having to wait for
the timer mechanism, is similar in concept to the proprietary PortFast mechanism, and only
operates on edge ports and point-to-point links. Other enhancements in RSTP, such as the synchronization
of root port information and the explicit forwarding authorization granted by
switches to other switches, have parallels with the UplinkFast and BackboneFast extensions.
534 Chapter 16  Using Spanning Tree with VLANs
It is likely that as time passes, greater emphasis will be placed by Cisco on the standardized
mechanisms of 802.1w rather than the proprietary extensions to 802.1D.
Those wishing to learn more about RSTP than is covered in the CCNP program
should visit www.cisco.com/warp/public/473/146.html.