Specifications

12-2
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-07
Chapter 12 Configuring STP
Understanding Spanning-Tree Features
• Spanning Tree and Redundant Connectivity, page 12-9
• Accelerated Aging to Retain Connectivity, page 12-9
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. 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.
When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a
network. The spanning-tree algorithm calculates the best loop-free path throughout a switched Layer 2
network. 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 the frames to construct a loop-free
path.
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 defines a tree with a root switch and a loop-free path from the root to all switches in the
Layer 2 network. 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.
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 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 media speed.
Supported Spanning-Tree Instances
The switch supports the per-VLAN spanning tree (PVST) and a maximum of 64 spanning-tree instances.
The switch also supports the PVRST, which uses RSTP to provide rapid convergence of the PVST, only
if your switch in running the enhanced software image (EI). For information about the PVRST, see the
“Spanning-Tree Instances Using RSTP” section on page 13-2. For information about how spanning tree
interoperates with the VLAN Trunking Protocol (VTP), see the “STP Configuration Guidelines” section
on page 12-11.
Bridge Protocol Data Units
The stable, active spanning-tree topology of a switched network is determined by these elements:
• The unique bridge ID (switch priority and MAC address) associated with each VLAN on each switch
• The spanning-tree path cost to the root switch
• The port identifier (port priority and MAC address) associated with each Layer 2 interface