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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter16 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
If a switch looses connectivity, it begins using the alternate paths as soon as the spanning tree selects a
new root port. By enabling UplinkFast with the spanning-tree uplinkfast global configuration
command, you can accelerate the choice of a new root port when a link or switch fails or when the
spanning tree reconfigures itself. The root port transitions to the forwarding state immediately without
going through the listening and learning states, as it wou ld w ith th e norma l sp an ni ng-t ree pr ocedu re s.
When the spanning tree reconfigures the new root port, other interfaces flood the netw ork with mu lticast
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 thi s p ar amete r is 15 0 p ac kets pe r
second). However, if you enter zero, station-learning frames are not generated, so the spanning-tree
topology converges more slowly after a loss of connectivity.
Note UplinkFast is most useful in wiring-closet switches at t h e ac c ess or edge o f the ne twork . It is not
appropriate for backbone devices. This feature might not be useful for other types of applicatio ns.
UplinkFast provides fast convergence after a direct link failure and achieves load balancing between
redundant Layer 2 links using uplink groups. An uplink group is a set of Layer 2 interfaces (per VLAN),
only one of which is forwarding at any given time. Specifically, 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 16-3 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 SwitchC that
is connected directly to Switch B is in a blocking state.
Figure16-3 UplinkFast Example Before Direct Link Failure
If Switch C detects a link failure on the currently active link L2 on the root port (a direct link failure),
UplinkFast unblocks the blocked port on Switch C and transitions it to the forwarding state without
going through the listening and learning states, as shown in Figure16-4. This change takes
approximately 1 to 5 se conds.
L1
L2 L3
Switch C
Switch A
(Root) Switch B
Blocked port
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