Description

This article explains the troubleshooting steps that we can perform for an interface that is administratively up, but the link is down.

Symptoms

If an interface stays down, you will be seeing the below log message in an SNMP-enabled switch.
 

mib2d[8791]: %DAEMON-4-SNMP_TRAP_LINK_DOWN: ifIndex 1096, ifAdminStatus up(1), ifOperStatus down(2), ifName et-2/0/19

User@switch> show interfaces terse et-2/0/19
Interface               Admin Link Proto    Local                 Remote    
et-2/0/19                up    down
et-2/0/19.0              up    down eth-switch

Below logs may also be seen on the device :-

Jul 9 01:52:13 device : %PFE-3: fpc0 qsfp-0/0/4 failed to mask LOS alarms
Jul 9 01:52:16 device : %PFE-3: fpc0 qsfp-0/0/5 failed to mask LOS alarms
Jul 9 01:52:25 device : %PFE-3: fpc0 qsfp-0/0/6 failed to mask LOS alarms
Jul 9 01:52:27 device : %PFE-3: fpc0 qsfp-0/0/7 failed to mask LOS alarms
Jul 9 01:52:34 device : %PFE-3: fpc0 qsfp-0/0/10 failed to mask LOS alarms
Jul 9 01:52:39 device : %PFE-3: fpc0 qsfp-0/0/11 failed to mask LOS alarms
Jul 9 01:52:51 device : %PFE-3: fpc0 qsfp-0/0/12 failed to mask LOS alarms
Jul 9 01:52:57 device : %PFE-3: fpc0 qsfp-0/0/13 failed to mask LOS alarms
Jul 9 01:53:03 device : %PFE-3: fpc0 qsfp-0/0/16 failed to mask LOS alarms
Jul 9 01:53:08 device : %PFE-3: fpc0 qsfp-0/0/17 failed to mask LOS alarms
Jul 9 01:53:15 device : %PFE-3: fpc0 qsfp-0/0/22 failed to mask LOS alarms
Jul 9 01:53:18 device : %PFE-3: fpc0 qsfp-0/0/23 failed to mask LOS alarms

Solution

Below are the steps one can follow to isolate hardware issues:
 

  1. Check if the cable is working .
  2. If the port has an optical transceiver (SFP/QFSP) inserted, we can check by replacing the transceiver with a known good working one. If the interface comes up with the working optic, then the issue might be with the optical transceiver.
  3. Remove the transceiver from the port, and insert it into a known good working port. In this case, if the interface comes up, the issue might be with the port.
  4. We can also check the port status by performing physical and software loopback tests.

Hard loopback / Physical loopback

- Create a physical loopback for the fiber-optic interface or a loopback plug for an RJ45 interface. If the interface comes up during loopback testing, then the SFP is good. If the interface stays down, you can try to check if the physical port has any internal connectivity issue.


Soft loopback / Local loopback

- When you create a local loopback, you create an internal loop on the interface being tested. A local loopback loops the traffic internally on that PIC and tests the interconnection of the PIC. If the interface stays down during this test, then the issue is with the physical port.

configure
edit interface <if_name> [ether-options | gigether-options]
set loopback
commit
 
Please follow the link for more details on the loopback test: https://www.juniper.net/documentation/us/en/software/junos/interfaces-ethernet/topics/topic-map/ethernet-10-40-100-gigabit-loopback-testing.html



If issue persists raise a JTAC case to get assistance. 

 

Modification History

2023-07-31 : Minor changes and additions. Added categories to be more generic.
2023-08-21 :- Added additional logs in symptoms
2023-10-05 : Added PTX, MX categories