The spanning tree network protocol provides a loop free topology for any LAN or bridged network. The Spanning Tree Protocol, which is also referred to as STP, is defined in the IEEE Standard 802.1D .

Its structure corresponds to that of the spanning tree in graph theory. 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 but 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 Virtual LANs exist, spanning tree can be deployed per VLAN. Cisco's name for this is per VLAN spanning tree (PVST and PVST+).

In 1998, the IEEE introduced an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP) or 802.1w.

In 2004 edition of 802.1D, STP is superseded by the Rapid Spanning Tree Protocol (RSTP).

External Links

802.1D-2004 standard

Cisco's version of 'Understanding STP'