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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter10 Configuring STP
Understanding Advanced STP Features
Understanding UplinkFast
Switches in hierarchical networks can be grouped into backbone sw itch es, d istri bution sw itche s, a nd
access switches. Figure 10-5 shows a complex network where distribution switches and access switches
each have at least one redundant link that STP blocks to prevent loops.
Figure10-5 Switches in a Hierarchical Network
If a switch looses connectivity, it begins using the alternate paths a s soon as STP sel ects a new r oot port .
By using STP UplinkFast, you can accelerate 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 w ou ld w it h n orma l STP pr oc edure s.
When STP reconfigures the new root port, other interfaces flood the network with multicast packets, one
for each address that was learned on the interface. You can limit these bursts of multicast traffic by
reducing the max-update-rate parameter (the default for th is p aram ete r is 15 0 packe ts per sec ond ).
However, if you enter zero, station-learning frames are not generated, so the STP topology converges
more slowly after a loss of connectivity.
Note UplinkFast is most useful in wiring-closet switches at the access or edge of the network. It is not
appropriate for backbone devices. This feature might not be useful for other type s of appli cations.
UplinkFast provides fast convergence after a direct link fai lure an d a ch iev es loa d b ala nci ng be twe en
redundant Layer 2 links using uplink groups. An uplink group is a se t of Layer 2 interfaces (per VLAN),
only one of which is forwarding at any given time. Speci fically, an uplink group consists of the root port
(which is forwarding) and a set of blocked ports, except for self-looping ports. The uplink group
provides an alternate path in case the currently forwarding link fails.
Figure 10-6 shows an example topology with no link failures. Switch A, the root switch, is connected
directly to SwitchB over link L1 and to Switch C over link L2. The Layer 2 interface on Switch C that
is connected directly to Switch B is in a blocking state.
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3500 XL 3500 XL 3500 XL
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Active link
Blocked link
Root bridge
Backbone switches
Distribution switches
Access switches
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