Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual page 158

Understanding How Loop Guard Works
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 9-6
A
3/2
Figure 9-6
• 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
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
9-6
Chapter 9
Triangle Switch Configuration with Loop Guard
3/1 3/1
3/1 3/2
C
Designated port
Root port
Alternate port
illustrates the following configuration:
We recommend that you enable loop guard on root ports and alternate root ports on access
switches.
Configuring Spanning Tree PortFast, UplinkFast, BackboneFast, and Loop Guard
B
3/2
Figure 9-6 shows loop guard in a
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