Configuring Loopback Detection

1 Configuration Notes and Constraints
2 Configuring Loopback Detection
3 Example for Configuring Loopback Detection
- 3.1 Networking Requirements
- 3.2 Procedure

Configuration Notes and Constraints

When configuring loopback detection, consider the following points:

The loopback detection message is sent in the native VLAN. When a loopback detection message is received, the device considers the source MAC address to determine if the message was sent from this device, without considering the VLAN tag.
Loopback detection can be enabled on a physical interface or a Link Aggregation Group (LAG) interface but cannot be enabled on the member interfaces of a LAG.
To enable the loopback detection function, users need to enable loopback detection BOTH globally and at the per-interface level.
Loopback detection is not supported in Multi-Chassis Link Aggregation (MLAG) scenarios.

Configuring Loopback Detection

Step 1 Enable loopback detection function globally.

set protocols loopback-detection enable <true | false>

Step 2 Enable loopback detection function on a specific interface.

set protocols loopback-detection interface <interface-name> enable <true | false>

Step 3 (Optional) Configure the loopback detection message transmission period.

set protocols loopback-detection message-interval <message-interval>

Step 4 (Optional) Clear the err-disable state of the loopback-detection interface.

run clear loopback-detection interface {<interface-name> | all}

Step 5 Commit the configuration.

commit

Step 6 View the configuration information and status information of loopback detection.

run show loopback-detection

Example for Configuring Loopback Detection

Networking Requirements

Figure 1. Loopback Detection Configuration Example

As shown in Figure 1, if there is a loop in the network connected to interface ge-1/1/1 of the device Switch, it will cause a broadcast storm, and communication on the Switch and the entire network may be affected.

Users hope to detect loops in the downstream network connected to the Switch and reduce the impact of the loop to the networks by blocking the downlink interface. Users can achieve this by enabling loopback detection function on ge-1/1/1 of the Switch. Once a loop is detected, interface state of ge-1/1/1 will be set to err-disable, resulting in the disconnection of the interface. This action helps to reduce the impact of the loop on the device and the network.

Procedure

Step 1 Enable loopback detection function globally.

admin@Switch# set protocols loopback-detection enable true

Step 2 Enable loopback detection function on a specific interface.

admin@Switch# set protocols loopback-detection interface ge-1/1/1 enable true

Step 3 Commit the configurations.

admin@Switch# commit

Step 4 View the configuration information and status information of loopback detection.

admin@Switch# run show loopback-detection 
Loopback-detection: enabled  
message-interval: 30s  
Interface       LBD Tx         Status                 From_Port 
----------      -----------    ------------           ----------- 
ge-1/1/1        true           LoopDetected           ge-1/1/1  
ge-1/1/2        false          Normal  
......  

admin@Switch# run show interface gigabit-ethernet xe-1/1/1 
Physical interface: ge-1/1/1, Enabled, error-discard True(LBD), Physical link is Down  
Interface index: 73, QSFP+ type: 40G_BASE_SR4, Mac Learning Enabled  
Port mode: access  
Description:  
…….

From the show result, we can see that,

Status of interface ge-1/1/1 is “LoopDetected” and “error-discard True(LBD), Physical link is Down”, indicating a loop is detected and the interface is disabled.
When a loopback is detected, the field From_Port indicates the sending interface of loopback detection packets.