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Chapter 13 Switch Features
13.7 Loop Guard Commands
Use these commands to configure the IES to guard against loops on the edge of your network. Loop
guard allows you to configure the IES to shut down a port if it detects that packets sent out on that
port loop back to the IES. While Spanning Tree Protocol (STP) can help prevent loops in the core of
your network, it cannot prevent loops that occur on the edge of your network.
Loop guard is handles loop problems on the edge of your network. This can occur when a port is
connected to a IES that is in a loop state. Loop state occurs as a result of human error. It happens
when two ports on a switch are connected with the same cable. When a switch in loop state sends
out broadcast messages the messages loop back to the switch and are re-broadcast again and
again causing a broadcast storm.
If a switch (not in loop state) connects to a switch in loop state, then it will be affected by the
switch in loop state in the following way:
• It will receive broadcast messages sent out from the switch in loop state.
• It will receive its own broadcast messages that it sends out as they loop back. It will then re-
broadcast those messages again.
The loop guard feature checks to see if a loop guard enabled port is connected to a switch in loop
state. This is accomplished by periodically sending a probe packet and seeing if the packet returns
on the same port. If this is the case, the IES will shut down the port connected to the switch in loop
state.
Loop guard can be enabled on xDSL ports.
Note: After resolving the loop problem on your network use the commands to re-activate
the disabled port.
This table describes the commands.
Table 82 Loop Guard Commands Summary
COMMAND
show interface dsl <slot/
portlist> loopguard
show interface dsl <slot/
portlist> loopguard policy
show interface dsl <slot/
portlist> loopguard recover-
time
clear interface dsl <slot/
portlist> loopguard
show interface dsl <slot/
portlist> loopguard uni-vlan
interface dsl <slot/portlist>
active
loopguard
118
DESCRIPTION
Displays whether loop guard is enabled or disabled and packet
statistics on the specified DSL ports.
Displays the loop guard policy on the specified DSL ports.
Displays the recover time for the dynamic loopguard policy on the
specified DSL ports.
Clears loop guard statistics (packet counter) on the specified DSL
ports.
Displays the unicast VLAN tag that the specified DSL ports use for the
loop guard detection packets.
Enters the sub-command mode for configuring the specified DSL ports. C
Enables the specified port(s) on the IES. Use this to manually re-
activate a port disabled by loop guard.
Enables loop guard.
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1
E
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1
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13
C
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IES4005M User's Guide
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Troubleshooting
