Cisco WS-C4003 Software Guide - Page 98

How Spanning Tree Protocols Work Chapter 7 Configuring Spanning Tree The Spanning Tree Protocol (STP) uses a distributed algorithm that selects one bridge of a redundantly connected network as the root of a spanning tree connected active topology. STP assigns roles to each port depending on what the port's function is in the active topology. Port roles are as follows: • Root-A unique 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. See the next section, How a Topology Is Created. In Ethernet networks, only one active path may exist between any two stations. Multiple active paths between stations can cause loops in the network. When loops occur, some switches recognize stations on both sides of the switch. This situation causes the forwarding algorithm to malfunction allowing duplicate frames to be forwarded. Spanning tree algorithms provide path redundancy by defining a tree that spans all of the switches in an extended network and then forces certain redundant data paths into a standby (blocked) state. At regular intervals the switches in the network send and receive spanning tree packets which they use to identify the active path. If one network segment becomes unreachable, or if spanning tree costs change, the spanning tree algorithm reconfigures the spanning tree topology and reestablishes the link by activating a standby path. Spanning tree operation is transparent to end stations, which do not detect whether they are connected to a single LAN segment or a switched LAN of multiple segments. How a Topology Is Created All switches in an extended LAN participating in a spanning tree gather information about other switches in the network through an exchange of data messages known as bridge protocol data units (BPDUs). This exchange of messages results in the following actions: • A unique root switch is elected for the spanning tree network topology. • A designated switch is elected for every switched LAN segment. • Any loops in the switched network are eliminated by placing redundant switch ports in a backup state; all paths that are not needed to reach the root switch from anywhere in the switched network are placed in STP-blocked mode. The topology of an active switched network is determined by the following: • The unique switch identifier (MAC address of the switch) associated with each switch • The path cost to the root associated with each switch port • The port identifier (MAC address of the port) associated with each switch port In a switched network, the root switch is the logical center of the spanning tree topology. A spanning tree protocol uses BPDUs to elect the root switch and root port for the switched network, as well as the root port and designated port for each switched segment. Software Configuration Guide-Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4 7-2 78-12647-02