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Software Configuration Guide—Release 12.2(25)EW
OL-6696-01
Chapter 15 Configuring STP Features
Overview of Root Guard
Overview of Root Guard
Spanning Tree root guard forces an interface to become a designated port, to protect the current root
status and prevent surrounding switches from becoming the root switch.
When you enable root guard on a per-port basis, it is automatically applied to all of the active VLANs
to which that port belongs. When you disable root guard, it is disabled for the specified port and the port
automatically goes into the listening state.
When a switch that has ports with root guard enabled detects a new root, the ports will go into
root-inconsistent state. Then, when the switch no longer detects a new root, its ports will automatically
go into the listening state.
Overview of Loop Guard
Loop guard helps prevent bridging loops that could occur because of a unidirectional link failure on a
point-to-point link. When enabled globally, loop guard applies to all point-to-point ports on the system.
Loop guard detects root ports and blocked ports and ensures that they keep receiving BPDUs from their
designated port on the segment. If a loop-guard-enabled root or blocked port stop receiving BPDUs from
its designated port, it transitions to the blocking state, assuming there is a physical link error on this port.
The port recovers from this state as soon as it receives a BPDU.
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. Figure 15-1 shows loop guard in a
triangular switch configuration.
Figure 15-1 Triangular Switch Configuration with Loop Guard
3/1
3/1
3/2
3/1 3/2
3/2
AB
C
Designated port
Root port
Alternate port
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