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Rapid Spanning Tree

Improve failover with the ues of Rapid Spanning Tree Protocol.

The venerable Spanning Tree Protocol, created by IBM back in the Dark Ages and standardized by the IEEE as 802.1D, has been a weak link from the beginning, but rapidly approached "galactically annoying" status in the '90s when network designers became enamored with VLAN trunking. The problem in a nutshell was that STP, which was designed with Source-Route Bridging in mind, had no mechanism to elegantly accommodate the various paths of multiple virtual networks traversing the same physical link. The result was adding a lot of confusion to the already painfully slow reconvergence time. To deal with these issues, some vendors created proprietary hacks to the protocol that had the effect of dramatically reducing the convergence time, but increasing complexity and by extension, hampering troubleshooting.

The good news, though, is that at the end of the last century, the IEEE proposed standards 802.1s and 802.1w, which were ratified in the 2002/2003 timeframe. These standards improve on the proprietary improvements by adding capabilities for multiple VLANs, and by cleaning up the base Spanning Tree algorithm, which essentially turns it into a connection-oriented protocol instead of the old method, where switches just broadcast BPDUs and waited for their timers to expire, with no confirmation of receipt. This is somewhat comparable to the difference between RIP and OSFP routing protocols in both the concept and the relative time to converge.

So, if you currently have a network that involves a lot of layer 2 connections between switches, and especially if you're doing a lot of VLAN trunking, you should put some serious thought into "Rapid Spanning Tree Protocol" (RSTP) as 802.1w is known. It's possible to improve your recovery time after a switch failure from 3-60 seconds, depending on how well you tweaked the timers, to 100-300 ms without any timer-tweaking required. In fact, the recovery using RSTP is faster than 802.1D with extensions like PortFast, BackboneFast and UplinkFast.

Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.

This was last published in June 2005

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