Understanding How Loop Guard Works - Cisco WS-C2948G-GE-TX Configuration Manual

Understanding How Loop Guard Works

Figure 8-5
Switch C
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. Loop guard
checks if a root port or an alternate root port receives BPDUs. If the port is receiving BPDUs, loop guard
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 per port with the set spantree guard loop command.
Note When 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.
triangle switch configuration.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
8-6
Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard
Adding a Switch in a Shared-Medium Topology
Switch A
(Root)
Blocked port
Switch B
(Designated Bridge)
Added switch
Figure 8-6 shows loop guard in a
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