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Understanding RSTP(802.1w)- Part1

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 Open Shortest 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.

Rapid Spanning 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.

Support of RSTP in Catalyst Switches

This table shows the support of RSTP in Catalyst switches, and the minimum software required for that support.

New Port States and Port Roles

The 802.1D is defined in these five different port states:

  • Disabled
  • listening
  • Learning
  • Blocking
  • forwarding

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, and so 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. Unfortunately, once in the forwarding state, there is no way to infer from the port state whether the port is root or designated. This contributes to demonstrate the failure of this state-based terminology. RSTP decouples the role and the state of a port to address this issue.

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