Understanding rapid spanning tree protocol

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Understanding Rapid Spanning Tree Protocol (802.1w)
Contents
Introduction Support of RSTP in Catalyst Switches New Port States and Port Roles Port States Port Roles New BPDU Format New BPDU Handling BPDU are Sent Every Hello-Time Faster Aging of Information Accepts Inferior BPDUs Rapid Transition to Forwarding State Edge Ports Link Type Convergence with 802.1D Convergence with 802.1wProposal/Agreement Sequence UplinkFast New Topology Change Mechanisms Topology Change Detection Topology Change Propagation Compatibility with 802.1D Conclusion Cisco Support Community - Featured Conversations Related Information

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Introduction
The 802.1D Spanning Tree Protocol (STP) standard was designed at a time when the recovery of connectivity after an outage within a minute or so was considered adequate performance. With the advent of Layer 3 switching in LAN environments, bridging now competes with routed solutions where protocols, such as OpenShortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP), are able to provide an alternate path in less time. Cisco enhanced the original 802.1D specification with features such as Uplink Fast, Backbone Fast, and Port Fast to speed up the convergence time of a bridged network. The drawback is that these

mechanisms are proprietary and need additional configuration. RapidSpanning Tree Protocol (RSTP; IEEE 802.1w) can be seen as an evolution of the 802.1D standard more than a revolution. The 802.1D terminology remains primarily the same. Most parameters have been left unchanged so users familiar with 802.1D can rapidly configure the new protocol comfortably. In most cases, RSTP performs better than proprietary extensions of Cisco without any additional configuration.802.1w can also revert back to 802.1D in order to interoperate with legacy bridges on a per-port basis. This drops the benefits it introduces. The new edition of the 802.1D standard, IEEE 802.1D-2004, incorporates IEEE 802.1t-2001 and IEEE 802.1w standards. This document provides information about the enhancements added by RSTP to the previous 802.1D standard.

Support of RSTP in CatalystSwitches
This table shows the support of RSTP in Catalyst switches, and the minimum software required for that support. RPVST+ (also known as PVRST+) Not available. 12.1(20)EA2 12.1(13)EA1 12.1(14)EA1 12.1(19)EA1 12.1(14)AX Not available. 7.5 12.1(19)EW Not available. 7.5

Catalyst Platform Catalyst 2900 XL / 3500 XL Catalyst 2940 Catalyst 2950/2955/3550 Catalyst 2970/3750 Catalyst 3560 Catalyst3750 Metro Catalyst 2948GL3/4908G-L3 Catalyst 4000/2948G/2980G (CatOS) Catalyst 4000/4500 (IOS) Catalyst 5000/5500 Catalyst 6000/6500 Catalyst 6000/6500 (IOS)

MST w/ RSTP Not available. 12.1(20)EA2 12.1(9)EA1 12.1(14)EA1 12.1(19)EA1 12.1(14)AX Not available. 7.1 12.1(12c)EW Not available. 7.1

12.1(11b)EX, 12.1(13)E, 12.2 12.1(13)E (14)SX

Catalyst 8500

Not available.

Not available.New Port States and Port Roles
The 802.1D is defined in these five different port states:


disabled listening learning blocking forwarding









See the table in the Port States section of this document for more information. The state of the port is mixed, whether it blocks or forwards traffic, and the role it plays in the active topology (root port, designated port, andso on). For example, from an operational point of view, there is no difference between a port in the blocking state and a port in the listening state. Both discard frames and do not learn MAC addresses. The real difference lies in the role the spanning tree assigns to the port. It can safely be assumed that a listening port is either designated or root and is on its way to the forwarding state....
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