Understanding How Loop Guard Works - Cisco WS-C4003 - Catalyst 4000 Chassis Switch Software Configuration Manual

Chapter 8 Configuring Spanning Tree PortFast, UplinkFast, and BackboneFast, and Loop Guard

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. 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.
Figure 8-6
Figure 8-6
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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:
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78-12647-02
Triangle Switch Configuration with Loop Guard
A
3/1 3/1
3/2
3/1 3/2
C
Designated port
Root port
Alternate port
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.
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.
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
B
3/2
Understanding How Loop Guard Works
Figure 8-6 shows loop guard in a
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