Description

This article explains one of the reasons why "CB X PLL Clock Loss of Lock" alarm occurs on an MX router.

 

 

Symptoms

We see a major alarm for the CB, "CB 1 PLL Clock Loss of Lock" which frequently sets and clears (alarm can be seen for any CB slot 0, 1, or 2).
 

user@router> show chassis alarms no-forwarding

3 alarms currently active
Alarm time               Class  Description
2023-11-30 01:41:44 GMT  Major  CB 1 PLL Clock Loss of Lock


We observed the following lines in the messages logs:


Nov 30 01:41:44  router alarmd[12793]: %DAEMON-4: Alarm set: CB id=671089014, color=RED, class=CHASSIS, reason=CB 1 PLL Clock Loss of Lock
Nov 30 01:41:44  router craftd[11499]: %DAEMON-4:  Major alarm set, CB 1 PLL Clock Loss of Lock 
 

Solution

PLL stands for Phase-Locked Loop. This is used to synchronize clock signals between the RE and the FPC.


The alarm "CB 1 PLL Clock Loss of Lock"  means that the signal called PLL between CB and RE is out of sync.  Since it is a message that is predicted when RE is started or restarted, it is not abnormal that it occurs temporarily. It is treated as a major alarm because it is a serious error, if it occurs in a case other than expected.

The cause of the PLL failure may be due to the hardware being in a state that is preventing it from responding properly. It could mean synchronization for clock is lost maybe due to RE being down on the CB. This could indicate hardware issue on the RE.

This behavior could be due to a transient hardware condition. Perform these steps to narrow down the issue and move to the next step if the previous step doesn't resolve the issue:
 

1. In syslogs, check if the alarm is logged as set -> clear -> set pattern
2. Reboot RE hosted on the affected CB
3. Swap the RE (to isolate CB or RE issue) with a spare and check if it comes online.
4. Perform physical reseat for CB.
5. If the alarms do not go away after re-seating the Control Board, then swap the CB with a spare working CB to isolate the issue with the CB or the chassis.


If the issue continues to exist, please open a JTAC case to troubleshoot further.

 


 

Modification History

6-12-2023: Initial Draft.
7-3-2025: Issue can be seeing on any CB.
7-31-2025: Minor update