3 vWICs in 2821 = errors? (I'm at a loss.)

Unanswered Question
Apr 15th, 2009

I learned yesterday that Cisco only truly supports 4 T1s in a 2821. I have 3 WICs in a 2821 (6 T1s) and it's been "working" fine. I noticed that I've been incurring CRC errors on ALL 6 of my interfaces. I've contacted the local telco, and they tested their line clean. I then had them come out and test from demarc to them, and it tested clean.

All of the connections are individual frame links that are bundled on my end. My question is:

If Cisco only supports 4 T1s coming into the router, and I have 6, will I a.) get CRC errors (normally line issue) and b.) will I see them only on the cards that's having a problem or will it be just local to the card that's having a problem.



I have this problem too.
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John Blakley Fri, 04/17/2009 - 12:03

The problem is fixed, and I'm posting here so everyone will know how I fixed it.

The telco tested everything fine, and it showed that the router was causing the errors. The problem is this:

When multiple vWICs are all set to network clock participate, they are all pulling the clock from the line. Since they were ALL doing this, it seemed that they were fighting because they were all also pulling from the backplane to get the timing.

I made one controller the primary, and then I set the other controllers to pull the clocking from that one and all of my errors went away.

controller t1 0/0/0

clock source line primary

controller t1 0/0/1

clock source line

controller t1 0/1/0

clock source line

controller t1 0/1/1

clock source line

network-clock-participate wic 0

network-clock-participate wic 1

network-clock-participate wic 2

network-clock-select 1 t1 0/0/0

The "network-clock-select" tells all of the other cards to get their clocking from t1 0/0/0. All of my errors went away and it resolved my issue.

HTH in the future,


george.inta Wed, 01/30/2013 - 15:42

Hi John,

After 6 hours of searching all the internet, trying to configure clock source independent, reconfiguring the mgcp gateway, i have finally found the solution here.This command "network-clock-select" should be written all over the web sites, even included by Cisco in their official documentation(because i didn't found it anywhere included).

Thank you,


Paolo Bevilacqua Thu, 01/31/2013 - 04:22

That is actually a subject that any reputable consultant, or certified Cisco partnerr, normally configure all the time.

Paolo Bevilacqua Wed, 04/15/2009 - 23:01

There is no limitation to the number of cards supported, your issue have a different cause.


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