Cisco WS-X6K-SUP1A-PFC= Software Guide - Page 158

Understanding How Loop Guard Works Chapter 9 Configuring Spanning Tree PortFast, UplinkFast, BackboneFast, and Loop Guard You can enable loop guard on a per-port basis. When you enable loop guard, it is automatically applied to all of the active instances or VLANs to which that port belongs. When you disable loop guard, it is disabled for the specified ports. Disabling loop guard moves all loop-inconsistent ports to the listening state. If you enable loop guard on a channel and the first link becomes unidirectional, loop guard blocks the entire channel until the affected port is removed from the channel. Figure 9-6 shows loop guard in a triangle switch configuration. Figure 9-6 Triangle Switch Configuration with Loop Guard A 3/1 B 3/1 3/2 3/2 3/1 3/2 C Designated port Root port Alternate port 55772 Figure 9-6 illustrates the following configuration: • Switches A and B are distribution switches. • Switch C is an access switch. • Loop guard is enabled on ports 3/1 and 3/2 on Switches A, B, and C. Use loop guard only in topologies where there are blocked ports. Topologies that have no blocked ports, which are loop free, do not need to enable this feature. Enabling loop guard on a root switch has no effect but provides protection when a root switch becomes a nonroot switch. Follow these guidelines when using loop guard: • You cannot enable loop guard on PortFast-enabled or dynamic VLAN ports. • You cannot enable PortFast on loop guard-enabled ports. • You cannot enable loop guard if root guard is enabled. Loop guard interacts with other features as follows: • Loop guard does not affect the functionality of UplinkFast or BackboneFast. • Do not enable loop guard on ports that are connected to a shared link. Note We recommend that you enable loop guard on root ports and alternate root ports on access switches. Catalyst 6000 Family Software Configuration Guide-Releases 6.3 and 6.4 9-6 78-13315-02