Cisco WS-C2960S-48TD-L Software Guide - Page 164

Configuring STP Chapter 6 Configuring the System Configuring Redundant Links By Using STP UplinkFast Switches in hierarchical networks can be grouped into backbone switches, distribution switches, and access switches. Figure 6-8 shows a complex network where distribution switches and access switches each have at least one redundant link that STP blocks to prevent loops. If a switch looses connectivity, the switch begins using the alternate paths as soon as STP selects a new root port. STP UplinkFast is a Cisco enhancement that accelerates 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 ports flood the network with multicast packets, one for each address that was learned on the port. 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 edge or access switches and might not be appropriate for backbone devices. Figure 6-8 Switches in a Hierarchical Network Backbone switches Root bridge 3500 XL 3500 XL Distribution switches 2900 XL 2900 XL 2900 XL 22037 2900 XL 2900 XL Active link Blocked link 2900 XL 2900 XL Access switches 6-36 Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide 78-6511-08