Understanding How Root Guard Works; Understanding How Loop Guard Works - Cisco 7604 Configuration Manual

Chapter 21 Configuring Optional STP Features

Understanding How Root Guard Works

The STP root guard feature prevents a port from becoming root port or blocked port. If a port configured
for root guard receives a superior BPDU, the port immediately goes to the root-inconsistent (blocked)
state.

Understanding How Loop Guard Works

Loop guard helps prevent bridging loops that could occur because of a uni-directional link failure on a
point-to-point link. When enabled globally, the 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 a receiving
BPDUs from its designated port, it transitions to the loop-inconsistent blocking state, assuming there is
a physical link error on this port. The port recovers from this loop-inconsistent 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.
triangle router configuration.
Figure 21-6 Triangle Switch Configuration with Loop Guard
Figure 21-6
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Enabling loop guard on a root router has no effect but provides protection when a root router becomes a
nonroot router.
When using loop guard, follow these guidelines:
OL-4266-08
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 router.
Loop guard is enabled on ports 3/1 and 3/2 on Switches A, B, and C.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
B
3/2
Understanding How Root Guard Works
Figure 21-6 shows loop guard in a
21-7