Understanding Uplinkfast - Cisco WS-C3550-12G Software Configuration Manual

Understanding Advanced STP Features

Understanding UplinkFast

Switches in hierarchical networks can be grouped into backbone switches, distribution switches, and
access switches.
each have at least one redundant link that STP blocks to prevent loops.
Figure 10-5 Switches in a Hierarchical Network
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If a switch looses connectivity, it begins using the alternate paths as soon as STP selects a new root 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 would with normal STP procedures.
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 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 wiring-closet switches at the access or edge of the network. It is not
Note
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.
Figure 10-6
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 3550 Multilayer Switch Software Configuration Guide
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Figure 10-5 shows a complex network where distribution switches and access switches
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Active link
Blocked link
shows an example topology with no link failures. Switch A, the root switch, is connected
Backbone switches
Root bridge
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Distribution switches
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Access switches
Chapter 10 Configuring STP
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