10-5
Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 10 Configuring STP
Understanding Basic STP Features
STP Timers
Table 10-2
describes the STP timers that affect the entire spanning-tree performance.
Creating the STP Topology
In
Figure 10-1
, 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 an STP recalculation to form a new topology with the ideal switch as the root.
Figure 10-1 STP 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 current 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 STP 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 10-2
Spanning Tree Protocol 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
DP
DP
RP
DP
DP
RP
DP
RP = Root Port
DP = Designated Port
DP
RP
DP
D
A
C
B
