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Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
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Chapter 16 Configuring Optional STP Features
Understanding How Loop Guard Works
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 16-6 shows loop guard in a
triangle switch configuration.
Figure 16-6 Triangle Switch Configuration with Loop Guard
Figure 16-6 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.
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 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.
• Enabling loop guard on ports that are not connected to a point-to-point link will not work.
• Root guard forces a port to be always designated as the root port. Loop guard is effective only if the
port is a root port or an alternate port. You cannot enable loop guard and root guard on a port at the
same time.
• Loop guard uses the ports known to spanning tree. Loop guard can take advantage of logical ports
provided by the Port Aggregation Protocol (PAgP). However, to form a channel, all the physical
ports grouped in the channel must have compatible configurations. PAgP enforces uniform
configurations of root guard or loop guard on all the physical ports to form a channel.
These caveats apply to loop guard:
–
Spanning tree always chooses the first operational port in the channel to send the BPDUs. If that
link becomes unidirectional, loop guard blocks the channel, even if other links in the channel
are functioning properly.
–
If a set of ports that are already blocked by loop guard are grouped together to form a channel,
spanning tree loses all the state information for those ports and the new channel port may obtain
the forwarding state with a designated role.
3/1 3/1
3/2
3/1 3/2
3/2
AB
C
Designated port
Root port
Alternate port
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