Cisco T1 Clocking Promble (The magic non-existent clock...)

I am trying to eastablish a T1 link between two building about 4 miles apart. (No, installing fiber is not an option and there is no line-of-sight.) My company is in a small town that seems to have only recently removed tin can and strings from its phone system.

This should be simple. Except of the DAC at the CO it, is essentially a direct link. We are using two Cisco 2651's with internal CSU/DSU's. We defaulted the config - Extended Super Frame, B8ZS, no FDL. We tried to supply clocking as directed by the telco. However, the two sites would NOT communicate when I configured one router to supply clocking. Only when we removed the clocking would the connection work. The link will stay up for 3 to 4 weeks at a time, but will then go through a period of frame losses that last from 3 to 4 hours. The phone company says it is my problem and that I need to supply clocking. However, the link only works when I do NOT supply clocking.

Any suggestions or ideas how to proceed.

BTW: ALL of my equipment (the routers and CSU/DSU cards) have been swapped out to eliminate bad equipment at my locations.

Thanks, Jim
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lrmooreConnect With a Mentor Commented:
Can you provide us with an example of the configuration that you use to provide the clocking?
When you DO provide clock on one side, what do you get with "show interface" ?
Have you tried PPP encapsulation vs the default HDLC?
Jim_McCartyAuthor Commented:
It turns out that the vendor problably had a problem. After opening multiple problem tickets and a great deal of complaining, we received a call from the vendor who asked that we "try internal clocking one more time." Suprisingly, it worked like a charm this time. Of course the vendor says they did nothing, but we used the same clocking statement we had used several time previously.

Here is the final config of the T1:

The main office router:

interface Serial0/1
 description Branch T1 Line
 ip address
 service-policy output VOIP
 encapsulation ppp
 service-module t1 clock source internal
 service-module t1 timeslots 1-24

The branch office router:

interface Serial0/0
 description Link to Main Office
 bandwidth 1544
 ip address
 service-policy output VOIP
 encapsulation ppp
 service-module t1 timeslots 1-24
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