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Chapter11 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Understanding UplinkFastSwitches in hierarchical networks can be grouped into backbone switch es, distribution switches, and
access switches. Figure11-2 shows a complex network where distribution switches and access switches
each have at least one redundant link that spanning tree blocks to prevent loops.
Figure11-2 Switches in a Hierarchical Network
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.
Note UplinkFast is most useful in wiring-closet switches at t he access or edge of the network. 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.
Active link
Blocked link
Root bridge
Backbone switches
Distribution switches
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