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3Com Switch 7750 Series
Command Reference Guide – MSTP Chapter 1
MSTP Configuration Commands
1-28
1.1.26 stp loop-protection
Syntax
stp loop-protection
undo stp loop-protection
View
Ethernet port view
Parameter
None
Description
Use the stp loop-protection command to enable the loop prevention function on the
current port.
Use the undo stp loop-protection command to restore the loop prevention function to
the default state on the current port.
By default, the loop prevention function is disabled.
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 congestions and 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 transit to forwarding state. This
may cause loops in the network.
The loop prevention function suppresses loops. With this function enabled, if link
congestions or link failures happen, a root port becomes a designated port, and the port
state becomes discarding. The blocked port also becomes designated port and the port
state becomes discarding (do not forward packets), and thereby loops can be
prevented.
Example
# Enable the loop prevention function on Ethernet1/0/1 port.
<3Com> system-view
System View: return to User View with Ctrl+Z.
[3Com] interface Ethernet1/0/1
[3Com-Ethernet1/0/1] stp loop-protection