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Understanding How Loop Guard Works
If a new switch is introduced into a shared-medium topology, BackboneFast is not activated.
shows a shared-medium topology in which a new switch is added. The new switch begins sending
inferior BPDUs that say it is the root switch. However, the other switches ignore these inferior BPDUs
and the new switch learns that Switch B is the designated bridge to Switch A, the root switch.
Figure 9-5
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 not 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 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.
triangle switch configuration.
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
9-6
Chapter 9
Configuring Spanning-Tree PortFast, 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 9-6
Figure 9-5
shows loop guard in a
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Troubleshooting
