Cisco Systems DOC-7814982 Creating the Spanning-Tree Topology, Spanning-Tree Interface States

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Catalyst 2950 Desktop Switch Software Configuration Guide

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Chapter 11 Configuring STP Understanding Spanning-Tree Features

Creating the Spanning-Tree Topology

In Figure 11-1, Switch A is elected as the root switch because the switch priority of all the switches is

set to the default (32768) and Sw itch A has the lowest MAC address. However, becaus e of traffic

patterns, number of forwarding interfaces, or link types, Switch A might not be the ideal root switch. By

increasing the priority (lowering the numerical value) of the ideal switch so that it becomes the root

switch , y ou fo r ce a s p an ni ng -tree recalculatio n to f orm a n ew topology with th e ide al s witc h as the r oo t.

Figure 11-1 Spanning-Tree Topology

When the spanning-tree topology is calculated based on default parameters, the path between source and

destination end stations in a switched network might not be ideal. For instance, connecting higher-speed

links to an interface that has a higher number than the root port can cause a root-port change. The goal

is to make the fastest link the root port.

For example, assume that one port on Switch B is a Gigabit Ethernet link and that another port on Switch

B (a 10/100 link) is the root port. Network traf fic might be more eff icient over the Gigabit Ethernet link.

By changing th e sp anning -t re e port p rior ity on the Gigabit Ethernet interface to a higher priority (lower

numerical value) than the root port, the Gigabit Ethernet interface becomes th e new root port.

Propagat ion delays can occur whe n protocol info rmat ion passes throug h a switche d LAN. As a result,

topology c ha nges c an take p lac e at different time s an d at different p laces in a s wit che d net work. W he n

an interface transition s directly f rom n onp articipation in the s panning -tre e topo logy to the for warding

state, it can create temporary data loops. Interfaces must wait for new topology information to propagate

through the switched LAN before starting to forward frames. They must allow the frame lifetime to

expire for for warde d fram es t hat have used the ol d to pology.

Each Layer 2 interface on a switch using spanning tree exists in one of these states:

•Blocking—The interface does not participate in frame forwarding.

•Listening—The first transitional state after the blocking state when the spanning tree determines

that the interface should participate in frame forwarding.

•Learning—The interface prepares to participate in frame forwarding.

•Forwarding—Th e inte rface forwa rds fram es.

•Disabled—The interf ace is not particip ating in spanning tr ee because of a shutdo wn port , no link on

the po rt, or no span ni ng-t ree inst an ce r unn ing on the po rt.

43568

RP DP

RP = Root Port

DP = Designated Port