Understanding Spanning-Tree Protocol
Spanning-Tree Protocol is a link management protocol that provides path redundancy while preventing undesirable loops in the network. For an Ethernet network to function properly, only one active path can exist between two stations.
Multiple active paths between stations cause loops in the network. If a loop exists in the network topology, the potential exists for duplication of messages. When loops occur, some switches see stations appear on both sides of the switch. This condition confuses the forwarding algorithm and allows duplicate frames to be forwarded.
To provide path redundancy, Spanning-Tree Protocol defines a tree that spans all switches in an extended network. Spanning-Tree Protocol forces certain redundant data paths into a standby (blocked) state. If one network segment in the Spanning-Tree Protocol becomes unreachable, or if Spanning-Tree Protocol costs change, the spanning-tree algorithm reconfigures the spanning-tree topology and reestablishes the link by activating the standby path.
Spanning-Tree Protocol operation is transparent to end stations, which are unaware whether they are connected to a single LAN segment or a switched LAN of multiple segments.
Election of the Root Switch
All switches in an extended LAN participating in Spanning-Tree Protocol gather information on other switches in the network through an exchange of data messages. These messages are bridge protocol data units (BPDUs). This exchange of messages results in the following:
- The election of a unique root switch for the stable spanning-tree network topology.
- The election of a designated switch for every switched LAN segment.
- The removal of loops in the switched network by placing redundant switch ports in a backup state.
The Spanning-Tree Protocol root switch is the logical center of the spanning-tree topology in a switched network. All paths that are not needed to reach the root switch from anywhere in the switched network are placed in Spanning-Tree Protocol backup mode. Table C-1 describes the root switch variables, that affect the entire spanning-tree performance.