T1 clocking

Unanswered Question
Sep 20th, 2009

Can someone tell me who typically provides the clocking on a T1 WAN link?

I have a Cisco 1841 and connected to a bonded T1 from Level3. The router is configured with internal clocking and appears to be working fine. Should I find out from the ISP if they're providing the clocking and change it accordingly?

I have this problem too.
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Giuseppe Larosa Sun, 09/20/2009 - 14:13

Hello Clifton,

usually clock is provided by the WAN provider.

Just to say the default setting for clock is line on cisco routers.

I would contact the provider.

Hope to help


Paolo Bevilacqua Sun, 09/20/2009 - 14:22

Yes you should, T1s usually are clocked from line and that is the best option.

Unfortunately often first level telco support doesn't really know and may say anything.

Richard Burts Mon, 09/21/2009 - 08:22

I recently had an experience with Qwest and clocking for T1s. We were experiencing problems with a couple of newly installed T1s and got Qwest involved. The support person that I was dealing with explained that if there was Qwest equipment terminating the circuit (CSU/DSU etc or T1 to them for Internet access) that they typically provide clocking but for circuits terminated on customer equipment (customer to customer connection) they typically do not provide clocking.

That was certainly different from what I expected based on the assumption that providers typically provide clocking on T1 circuits. But in this case clock source line caused problems and clock source internal made the circuit work.

So if Clifton's circuit is working fine then I do not see much need to talk to the provider.



publigroupe Mon, 09/21/2009 - 08:57

Hey Rick,

Thanks for the post.

The WIC cards on my Cisco 1841 are acting as the CSU/DSU. I did contact Level3 and they told me that they're providing the clocking. I've subsequenly configured my router to clock souce line. I hope there wont be any problem.


Richard Burts Mon, 09/21/2009 - 09:25


If you checked with Level3 and they tell you that they are providing clock, then configuring your router for clock source line is the right thing to do. As long as they provided correct information to you then I would not expect any problem.



Wilson Samuel Mon, 09/21/2009 - 09:28

By the way I never explicitly configured Clocking on T1s whenever I deployed, are they all under some risk?


Richard Burts Mon, 09/21/2009 - 10:55

As Giuseppe points out in his post, the default for Cisco T1 integrated controllers (controller incorporated into the serial card) is for clock source line. So assuming that your T1s are all receiving clocking from the provider there should not be some risk.



viyuan700 Mon, 09/21/2009 - 12:25

"I did contact Level3 and they told me that they're providing the clocking"

As per my knowledge provider donot provide any clock means provider dont have to configure anything extra. Clock is ALWAYS there in ANY T1/E1 circuit.

As per SONET/SDH standard when you set your T1 card to inline it synchronize itself(your T1 card)to the SONET/SDH box of the provider.

When you dont set to inline then your T1 card and SONET box are generating excellent T1 signal but they are not synchronized. Means you and me have 2 excellent clock but we didnot set them at same time.


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