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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter10 Configuring STP
Understanding Advanced STP Features

Accelerated Aging to Retain Connectivity

The default for aging dynamic addresses is 5 minutes, the default setting of the mac-address-table
aging-time global configuration command. However, an STP reconfiguration c an c ause ma ny sta tion
locations to change. Because these stations could be unreachable for 5 minutes or more during a
reconfiguration, the address-aging time is accelerated so th at station addresses can be dropped from the
address table and then relearned. The accelerated aging is the same as the forward-delay parameter value
(spanning-tree vlan vlan-id forward-time seconds global configuration command) when STP
reconfigures.
Because each VLAN is a separate spanning-tree instance, the switch accelerates aging on a per-VLAN
basis. An STP reconfiguration on one VLAN can cause the dynamic addresse s learned on that VLAN to
be subject to accelerated aging. Dynamic addresses on other VLA Ns c a n be un affect ed a nd rema in
subject to the aging interval entered for the switch.
Understanding Advanced STP Features
This section describes how advanced STP features work. It includes this information:
•Understanding Port Fast, page 10-10
•Understanding BPDU Guard, page 10-11
•Understanding UplinkFast, page 10-12
•Understanding Cross-Stack UplinkFast, page 10-13
•Understanding BackboneFast, page 10-18
•Understanding Root Guard, page 10-20
•Understanding EtherChannel Guard, page 10-20
For configuration information, see the “Configuring Advanced STP Features” section on page 10-32.

Understanding Port Fast

Port Fast immediately brings an interface configured as a La ye r 2 a cce ss p ort to t he for w ardi ng st ate
from a blocking state, bypassing the listening and learning stat es. You can use Port Fast on Laye r 2
access ports connected to a single workstation or server, as shown in Figure 10-4, to allow those devices
to immediately connect to the network, rather than waiti ng for ST P to c onverge.
If the interface receives a BPDU, which should not happen if the interface is connected to a single
workstation or server, STP puts the port in the blocking state. An interface with Port Fast enabled goes
through the normal cycle of STP status changes when the switch is restarted. For more information, see
the “Configuring Port Fast” section on page 10-32.
Note Because the purpose of Port Fast is to minimize the time access ports must wait for STP to converge,
it is effective only when used on access ports. If you enable Port Fast on a port connecting to another
switch, you risk creating a spanning-tree loop.