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8-6
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
—
Release 8.1
78-15486-01
Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop
Understanding How Loop Guard Works
Figure 8-5
Adding a Switch in a Shared-Medium Topology
Understanding How Loop Guard Works
Unidirectional link failures may cause a root port or alternate port to become designated as root if
BPDUs are absent. Some software failures may introduce temporary loops in the network. The loop
guard feature checks if a root port or an alternate root port receives BPDUs. If the port is receiving
BPDUs, the loop guard feature puts the port into an inconsistent state until it starts receiving BPDUs
again. Loop guard isolates the failure and lets spanning tree converge to a stable topology without the
failed link or bridge.
You can enable loop guard on a per-port basis with the set spantree guard loop command.
Note
Provided that you are in MST mode, you can set all the ports on a switch with the set spantree
global-defaults loop-guard command.
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 8-6
shows loop guard in a
triangle switch configuration.
Switch A
(Root)
Switch C
Switch B
(Designated Bridge)
Added switch
11245
Blocked port