Specifications

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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide

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Chapter 12 Configuring STP

Understanding Spanning-Tree Features

Spanning-Tree Timers

Table 12-2 describes the timers that affect the entire spanning-tree performance.

Creating the Spanning-Tree Topology

In Figure 12-1, Switch A is elected as the root switch because the switch priority of all the switches is

set to the default (32768) and Switch A has the lowest MAC address. However, because 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, you force a spanning-tree recalculation to form a new topology with the ideal switch as the root.

Figure 12-1 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 traffic might be more efficient over the Gigabit

Ethernet link. By changing the spanning-tree port priority on the Gigabit Ethernet interface to a higher

priority (lower numerical value) than the root port, the Gigabit Ethernet interface becomes the new root

port.

Table 12-2 Spanning-Tree Timers

Variable Description

Hello timer Determines how often the switch broadcasts hello messages to other switches.

Forward-delay timer Determines how long each of the listening and learning states last before the interface begins

forwarding.

Maximum-age timer Determines the amount of time the switch stores protocol information received on an interface.

43568

RP DP

RP = Root Port

DP = Designated Port