3Com Switch 4500 Family Manual page 223

Parameters
interface-list: Ethernet port list. You can specify multiple Ethernet ports by providing this argument in the
form of interface-list = { interface-type interface-number [ to interface-type interface-number ] } &<1-10>,
where &<1-10> means that you can provide up to 10 port indexes/port index ranges for this argument.
Description
Use the stp loop-protection command to enable the loop guard function on the current port. Use
the undo stp loop-protection command to restore the loop guard function to the default state on
the current port.
Use the stp interface loop-protection command to enable the loop guard function on specified
ports in system view. Use the undo stp interface loop-protection command to restore the default
state of the loop guard function on specified ports in system view.
By default, the loop guard function is disabled on the port.
A switch maintains the states of the root port and other blocked ports by receiving and processing
BPDUs from the upstream switch. These BPDUs may get lost because of network congestion or
unidirectional link failures. If a switch does not receive BPDUs from the upstream switch for a certain
period, the switch selects a new root port; the original root port becomes a designated port; and the
blocked ports turn to the forwarding state. This may cause loops in the network.
The loop guard function suppresses loops. With this function enabled, if link congestions or
unidirectional link failures happen, a root port becomes a designated port, and the port turns to the
discarding state. The blocked port also becomes the designated port and the port turns to the
discarding state, that is, the port does not forward packets and thereby loops can be prevented.
You are recommended to enable loop guard on the root port and alternate port of a non-root bridge.
Loop guard, root guard, and edge port settings are mutually exclusive. With one of these functions
enabled on a port, any of the other two functions cannot take effect even if you have configured it
on the port.
Examples
# Enable the loop guard function on Ethernet 1/0/1.
Enable the loop guard function on Ethernet 1/0/1 in Ethernet port view.
<Sysname> system-view
System View: return to User View with Ctrl+Z.
[Sysname] interface Ethernet 1/0/1
[Sysname-Ethernet1/0/1] stp loop-protection
Enable the loop guard function on Ethernet 1/0/1 in system view.
<Sysname> system-view
System View: return to User View with Ctrl+Z.
[Sysname] stp interface Ethernet 1/0/1 loop-protection
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