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Spanning-tree recalculation / Rapid spanning-tree protocol

Spanning-tree recalculation
7.2.6 This page will describe the convergence of a spanning-tree network.
A switched internetwork has converged when all the switch and bridge ports are in either the forwarding or blocking state. Forwarding ports send and receive data traffic and BPDUs. Blocking ports only receive BPDUs.
When the network topology changes, switches and bridges recompute the spanning-tree and cause a disruption in network traffic.  
Convergence on a new spanning-tree topology that uses the IEEE 802.1d standard can take up to 50 seconds. This convergence is made up of the max-age of 20 seconds, plus the listening forward delay of 15 seconds, and the learning forward delay of 15 seconds.
The Lab Activities will show students how to create and verify a basic switch configuration.
The next page will introduce the Rapid Spanning-Tree Protocol.














Rapid spanning-tree protocol
7.2.7 This page will describe the Rapid Spanning-Tree Protocol.
The Rapid Spanning-Tree Protocol is defined in the IEEE 802.1w LAN standard. The standard and protocol introduce new features:
  • Clarification of port states and roles
  • Definition of a set of link types that can go to forwarding state rapidly
  • Concept of allowing switches in a converged network to generate BPDUs rather than relaying root bridge BPDUs
The blocking state of a port is renamed as the discarding state. The role of a discarding port is that of an alternate port. The discarding port can become the designated port if the designated port of the segment fails.
Link types have been defined as point-to-point, edge-type, and shared. These changes allow rapid discovery of link failure in switched networks.
Point-to-point links and edge-type links can go to the forwarding state immediately.
Network convergence should take no longer than 15 seconds with these changes.
The Rapid Spanning-Tree Protocol, IEEE 802.1w, will eventually replace the Spanning-Tree Protocol, IEEE 802.1d.
This page concludes this lesson. The next page will summarize the main points from this module.

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