11-2
Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 11 Configuring STP
Unders tanding Spanni ng-Tree Fea tures
•Spanning Tree and Redun da nt Co nnec tivity, page 1 1- 8
•Accelerated Aging to Retain Connectivity, page 11-9

STP Overview

STP is a Layer 2 link management protocol that provides path redundancy while preventing loops in the
network . For a Layer 2 Ethe rnet net work to func tion prop erly, only one active path can exist bet ween
any two stations. Spa nning -tree oper ation is transpa rent to end st ation s, which cannot detec t whethe r
they are connected to a single LAN segment or a switched LAN of multiple segments.
When you c rea te fault -tol era nt in tern etwor ks, you must have a lo op-fre e path be twee n a ll n ode s in a
network. Th e sp an ning- t ree a lgor ith m cal cul ate s t he best loop -f ree pa th t hr ough out a swit che d Laye r 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, bu t 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 a lso learn end-s tation MAC addre sses o n
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 fo rces redundant data paths into a standby (bloc ked) st ate. If a networ k
segment in the spanning tree fails and a redundant path exists, the spanning-tree algorithm recalculates
the spanni ng-tree topology and a ctiva tes the stan dby 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 t he forwardi ng state and which is put in the bl ocking sta te. The port
priorit y value re prese nts the loc ation of an inter face i n the net work topo l ogy and h ow well i t is loca te d
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.
For information about how spanning tree interoperates with the VLAN Trunking Protocol (VTP), see the
“STP Configuration Guidelines” sect ion on page 11-10.

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
When the switches in a network are powered up, each functions as the root switch. Each switch sends a
configurati on BPDU throug h all of its por ts. The BPD Us comm unicate an d compute the spanning -tree
topolo gy. Ea ch configu rati on B P DU co ntai ns thi s infor ma tion:
•The unique bridge ID of the switch that the sending switch identifies as the root switch
•The spanning-tree path cost to the root
•The br id ge ID of t he se nd ing switc h