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Spanning tree

The Spanning tree network protocol provides a loop free topology for any LAN or bridged network. Networks must have only one path to any destination active at any one point in time to avoid the same frame arriving at the destination multiple times, causing dysfunction. The minimum spanning tree algorithm ensures that if multiple paths exist to the same destination then all bar one will be blocked.

Redundant networks can be built in this way, however the latency of spanning tree decisions must be taken into account as by default the maximum time for recalculations of the minimum spanning tree is 30 seconds. During this time no network traffic will be able to pass through ports involved in the recalculations.

A root bridge is automatically selected by the algorithm as the root of the minimum spanning tree. As with most networking protocols this is highly configurable and therefore any networking device could be forced into this role.

Ports have five states:

  • Listening
  • Learning
  • Forwarding
  • Blocking
  • Disabled

Bridge protocol data units (BPDUs) are the frames which carry the spanning tree protocol information.

In ethernet switched environments where multiple VLANs exist, spanning tree can be deployed per VLAN. Cisco call this per VLAN spanning tree.

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