Spanning tree (networks)
The spanning tree network protocol provides a loop free topology for any bridged LAN. The Spanning Tree Protocol, which is also referred to as STP, is defined in the IEEE Standard 802.1D.
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Protocol operation
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 (The switches in a network will carry out elections to decide which one is the root bridge. The switch with the lowest priority wins the election and becomes the root bridge. In the event that all switches have the same priority, then the bridge ID will then be compared and the one again with the lowest wins the elections. In the event that another switch is added to the network where a root bridge is already elected then no elections are held even if the switch has the lowest priority). 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.
Evolutions and extensions
Per-VLAN Spanning Tree (PVST)
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+ which is the default protocol used by Cisco switches).
Rapid Spanning Tree Protocol (RSTP)
In 1998, the IEEE introduced an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP) or 802.1w. In the 2004 edition of 802.1D, STP is superseded by the Rapid Spanning Tree Protocol (RSTP).
Multiple Spanning Tree Protocol (MSTP)
The 2003 revision of the standard also rolled in the Multiple Spanning Tree Protocol (MSTP) originally defined in IEEE 802.1s and later merged into IEEE 802.1Q-2003
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External Links
- ANSI/IEEE 802.1D-2004 standard
- Cisco's version of 'Understanding STP'
- RFCs
- RFC 2674-1999, proposed standard, Definitions of Managed Objects for Bridges with Traffic Classes, Multicast Filtering and Virtual LAN Extensions
- RFC 1525-1993, - SBRIDGEMIB, proposed standard, Definitions of Managed Objects for Source Routing Bridges
- RFC 1493-1993 - BRIDGEMIB, draft standard, Definitions of Managed Objects for Bridges