Cisco Catalyst 2975 Software Configuration Manual page 480

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Understanding Optional Spanning-Tree Features

If a switch loses 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 would with the normal spanning-tree procedures.

When the spanning tree 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 this parameter is 150 packets per

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.

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 types of applications.

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.

directly to Switch B 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.

Catalyst 2975 Switch Software Configuration Guide

Switches in a Hierarchical Network

Blocked link

shows an example topology with no link failures. Switch A, the root switch, is connected

Backbone switches

Distribution switches

Access switches

Configuring Optional Spanning-Tree Features

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