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4 Wire CSU/DSU

I have and 2611 and a 2524 connected over a 56k DDS circuit terminated into two 4 wire CSU/DSU's respectively. The line status changes to down constantly every day for the past week. Upon further inspection, the 2611 is showing quite a few CRC and input errors on the serial interface whereas the 2524 shows only a few. A debug serial shows the mineseen and yourseen counters incrementing correctly but every 10 min. or so will show a keepalive mismatch, then shows the line going down then back up. The keepalives on both ends are set to 10 sec. each. Just for info. the encap. is set to HDLC. Im pulling my hair out over this(what hair is left anyway). Any help would be appreciated. Thanks.

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Cisco Employee

Re: 4 Wire CSU/DSU

Have you seen this doc? It is very helpful...

http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1915.htm

Also, it looks like you are having errors on you dds circuit. What appears to be happening is the when you have a CRC on a keepalive message, the router will resend the keepalive with the previous sequence number. When the other router sees the same sequence number twice in a row, it shows a keepalive mismatch. If it is happening about every 10 minutes, I would suggest looping the local CSU on each side back towards the router to verify the integrity of the routers. Then, you can loop to CSU's towards the far routers and test again.

If the loops run clean for a while, you might have a timing problem. Verify how the CSU's are being timed, and check with the telco if the DDS circuit is being timed or not. Based on their answer, you will need to set the timing on the CSU's differently. The key point is NOT to have multiple timing sources.

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