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Catalyst 2900 Series XL and Catalyst 3500 SeriesXL Software Configuration Guide
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Chapter 6 Configuring the System
Configuring STP
Configuring Redundant Links By Using STP UplinkFast
Switches in hierarchical networks can be grouped into backbone switches, distribution switches, and
access switches. Figure 6-8 shows a complex network where distribution switches and access switches
each have at least one redundant link that STP blocks to prevent loops.
If a switch looses connectivity, the switch begins using the alternate paths as soon as STP selects a new
root port. STP UplinkFast is a Cisco enhancement that accelerates the choice of a new root port when a
link or switch fails or when STP reconfigures itself. The root port transitions to the forwarding state
immediately without going through the listening and learning states, as it would with normal STP
procedures.
When STP reconfigures the new root port, other ports flood the network with multicast packets, one for
each address that was learned on the port. You can limit these bursts of multicast traffic by reducing the
max-update-rate parameter. The default for this parameter is 150 packets per second. However, if you
enter zero, station-learning frames are not generated, so the STP topology converges more slowly after
a loss of connectivity.
UplinkFast is most useful in edge or access switches and might not be appropriate for backbone devices.
Figure 6-8 Switches in a Hierarchical Network
3500 XL 3500 XL
2900 XL 2900 XL 2900 XL
2900 XL 2900 XL 2900 XL 2900 XL
Active link
Blocked link
Root bridge
Backbone switches
Distribution switches
Access switches
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