Summary

Transparent bridges and switches have three main functions: learn, forward, and remove loops. Cisco switches use the 802.1D protocol to remove loops in Layer 2 networks. BPDUs are used to discover the topology of the network, elect a root switch, and notify other switches of topology changes. BPDU's hellos are generated every 2 seconds.

STP elects a root switch, which is switch with the lowest bridge ID (priority + MAC address). Each switch chooses the best port to reach the root, called a root port: This is the port with the lowest accumulated path cost to the root. Each segment has one designated port on one switch, which is used to forward traffic to and from the segment. The switch with the lowest accumulated path cost is chosen. All root and designated ports will move from a blocking or listening state to learning and then forwarding. All other ports remain in a blocked state. It can take between 30 50 seconds for convergence to take place.

Non-Cisco switches use CST on 802.1Q trunks. Cisco switches support PVST when connected to other Cisco switches on ISL or 802.1Q trunks. In PVST, each VLAN has its own STP components: root switch, BPDUs, priorities, and port costs.

STP is enabled, by default, for all VLANs on Cisco switches. Use the spanning-tree priority command to influence which switch will become the root switch. Use the show spanning-tree command to view your STP components and operation.