14-2
Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
78-11380-10
Chapter14 Configuring STP
Understanding Spanning-Tree Features
•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 page14-1 1.
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 det ect whe the r th ey are c onn ect ed
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 ne twork as the
root of the spanning tree. The algorithm calculates the best loop-free path through a swi tched La yer 2
network by assigning a role to each port based on the role of the por t in th e ac tive topolo gy:
•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-t ree al gorithm r eca lcu lat es t he
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.