Switches can detect only external loops that occur on a single interface. After external loop detection is enabled, the switch sends packets periodically to check whether an external loop occurs on an interface. When a loop is found on an interface, the switch performs the specified action on the interface. In versions earlier than V200R002, the switch sets the interface state to blocking by default. In V200R002 and later versions, the switch sets the interface state to shutdown by default.
Usage Scenario
Generally, single-interface loop detection is used on downlink interfaces of newly deployed switches to help field engineers discover incorrect cable connections.
It is recommended that you set the action for interfaces with loops to block.
Configuration Procedure
After you enable loop detection globally, this function is enabled on all interfaces.
[Quidway] loopback-detect enableModular switches of V200R001 and later versions support loop detection in eight VLANs on an interface.
Fixed switches of V100R005 and later versions support loop detection in eight VLANs on an interface. In addition to trap, shutdown, and block, the action for interfaces with loops can be set to nolearn (stop learning MAC addresses).
The following configuration is performed on fixed switches:
[Quidway-Ethernet0/0/1] loopback-detect packet vlan 20 21 22 23 24 25 26 27
[Quidway-Ethernet0/0/1] loopback-detect action nolearn
Modular switches of V200R001 and later versions and fixed switches of V100R005 and later versions can generate loop traps, and the traps contain VLANs where loops have occurred.
The following is an example of loop trap:
#Jan 1 2008 06:43:54-08:00 Quidway LDT/4/Porttrap:OID1.3.6.1.4.1.2011.5.25.174.3.3 Loopback does exist on interface(5) Ethernet0/0/1 ( VLAN 20 ) , loopback detect status: 4.(1:normal; 2:block;3:shutdown; 4:trap; 5:nolearn)
Precautions
Loop detection is an auxiliary tool and consumes system resources. When loop detection is complete, run the undo loopback-detect enable command to disable this function.
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