Spanning Tree Protocol Convergence

By Dan DiNicolo, May 28th, 2006 Posted in CCNA Study Guide Chapter 03. Subscribe to our RSS Feed

While our network is now loop free, STP keeps working away. BPDUs are still sent out at 2-second intervals to be sure that things are how they should be. If at any point a bridge doesn’t hear from another bridge, the topology will need to be recalculated. For example, let’s say that bridge B fails. Bridge C will stop receiving BPDUs on its blocked port. Once 20 seconds have passed without receiving a BPDU from Bridge B, Bridge C will:

• First go into a listening state for 15 seconds. During the listening state, a bridge is examining BPDUs sent by other bridges.
• Then go into a learning state for 15 seconds. During this time, the bridge is building the MAC address table for the connected segment. Remember that it was in a blocked state previous to this.

After these stages, Bridge C will become the new designated bridge for segment BC, and will begin forwarding frames. Note that convergence took about 50 seconds to complete – 20 seconds waiting for a BPDU, plus 15 seconds listening and 15 seconds learning. The time during which a switch is listening and learning is referred to as the Forward Delay.

While a number of BPDUs are passed back and forth while a Spanning Tree topology is being calculated, in truth calculating a Spanning Tree topology is really no more than a three-step process:

1. Elect the Root Bridge
2. Elect a Root Port on each non-Root Bridge
3. Elect one Designated Port on each network segment.

Once these steps are completed, a network should be loop free. However, you should also recall that while listening or learning, ports are not forwarding frames. A network is converged once all bridges have switched to a forwarding or blocking state.

Written by Dan DiNicolo - Visit Website

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