Cisco 2950 Software Configuration Guide - Page 296

Understanding Spanning-Tree Features Chapter 14 Configuring STP • Spanning-Tree Interoperability and Backward Compatibility, page 14-10 • STP and IEEE 802.1Q Trunks, page 14-10 For configuration information, see the "Configuring Spanning-Tree Features" section on page 14-11. For information about optional spanning-tree features, see Chapter 16, "Configuring Optional Spanning-Tree Features." STP Overview STP is a Layer 2 link management protocol that provides path redundancy while preventing loops in the network. For a Layer 2 Ethernet network to function properly, only one active path can exist between any two stations. Multiple active paths among end stations cause loops in the network. If a loop exists in the network, end stations might receive duplicate messages. Switches might also learn end-station MAC addresses on multiple Layer 2 interfaces. These conditions result in an unstable network. Spanning-tree operation is transparent to end stations, which cannot detect whether they are connected to a single LAN segment or a switched LAN of multiple segments. The STP uses a spanning-tree algorithm to select one switch of a redundantly connected network as the root of the spanning tree. The algorithm calculates the best loop-free path through a switched Layer 2 network by assigning a role to each port based on the role of the port in the active topology: • Root-A forwarding port elected for the spanning-tree topology • Designated-A forwarding port elected for every switched LAN segment • Alternate-A blocked port providing an alternate path to the root port in the spanning tree • Backup-A blocked port in a loopback configuration Switches that have ports with these assigned roles are called root or designated switches. Spanning tree forces redundant data paths into a standby (blocked) state. If a network segment in the spanning tree fails and a redundant path exists, the spanning-tree algorithm recalculates the spanning-tree topology and activates the standby path. Switches send and receive spanning-tree frames, called bridge protocol data units (BPDUs), at regular intervals. The switches do not forward these frames but use them to construct a loop-free path. BPDUs contain information about the sending switch and its ports, including switch and MAC addresses, switch priority, port priority, and path cost. Spanning tree uses this information to elect the root switch and root port for the switched network and the root port and designated port for each switched segment. When two interfaces on a switch are part of a loop, the spanning-tree port priority and path cost settings determine which interface is put in the forwarding state and which is put in the blocking state. The spanning-tree port priority value represents the location of an interface in the network topology and how well it is located to pass traffic. The path cost value represents the media speed. 14-2 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 78-11380-10