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Clock Slips with Multiple PRI's

Hello, We are seeing clock slips on one of our PRI's. We have two PRI's from same carrier coming in to a 2951 router. The two PRI's are provided by Lightpath (PRI hand-off from fiber) via two separate Adtran 900e gateways. WIC's are both onboard the 2951, not in a network module. One of the PRI's (0/0/0) never has any errors, the other one (0/1/0) continuously increases. Both PRI's are set to receive their clock source from line.



Here is part of the config:



network-clock-participate wic 0
network-clock-participate wic 1
network-clock-select 1 T1 0/0/0
network-clock-select 2 T1 0/1/0

controller T1 0/0/0
 cablelength long 0db
 pri-group timeslots 1-24 service mgcp
 description <<MAIN LIGHTPATH PRI >>

controller T1 0/1/0
 cablelength long 0db
 pri-group timeslots 1-24 service mgcp
 description <<2nd LIGHTPATH PRI >>


0/1/0 continually gets slip errors. We are also experiencing some voice issues with that T1 - namely some one-way audio calls. I think the slip errors might be to blame, but I am not sure how to get rid of them. From everything I've read my config looks OK..  I did try setting the "bad" PRI to clock source internal and removed the network-clock-select 2 T1 0/1/0 line.. But that did not help.



These problems seem to have started after an IOS upgrade to 15.6(3)M5, but it's difficult to tell because we also did a Callmanager upgrade. However, the Slip errors bother me, I just don't think there should be any.  I would welcome any help or thoughts on this issue. Thanks in advance.


Nipun Singh Raghav
Cisco Employee

What happens when you swap the lines ? Do the slips follow the line or stay confined to the port irrespective of which line is plugged in ?
Also, from a Telco perspective are both the lines in their own clocking domains or is one primary while the other is secondary ?

Nipun Singh Raghav
"We cannot solve our problems with the same thinking we used when we created them"



I swapped the PRI's the other day to see what would happen.. The errors DID NOT follow the PRI, but instead remained with the interface and continued to tick up after I cleared them. Does anyone have an further thoughts on this? I am thinking that perhaps the Telco does not have the clocking configured in the best way on the two Adtrans. As I mentioned, the two Adtrans are both configured to receive clocking from an Internal source. So, perhaps they should be configured in a primary/secondary configuration instead?  Any further advice would be appreciated.

Thank you.


So I spoke with the telco last week and they said that each of the two Adtran’s clocking source is set to “Internal.” Maybe that is the problem? They could be slightly out of synch. Should I ask them to try changing that so that one is primary and one is secondary?  Would that be the preferred set-up for two PRI’s from the same provider?


I have not been able to test swapping the PRI’s yet. I need to do that after hours. But I will try that when I can. Thank you.

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