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New Member

Serial line problem


We have a remote site(2821 router) that is connected to our main site(1721 router) via 512k leased line. The line sometimes goes down and up for 1 - 2 minutes a day, sometimes it remains fine for a complete day, and sometimes it keeps on going up and down for 6 - 8 hours, mostly in after office hours where the load is virtually zero.

Now I left office Wednesday at 4.00 p.m. and found that the link was fine until friday morning (5.00 a.m.) where it flapped twice and then remained down.

I can see that there are a lot of CRC errors, interface resets, frame, aborts in the show int s0 output. The telco have visited us twice and it seems after he resets the modem or powers it off this problem goes away and then resurfaces after sometime (hours), even after the modem at our site was replaced. There are no alarms on the remote site modem.

I am wondering what the problem can be, I am planning to replace the router at our side and see if this changes anything, I do not know if clocking is the issue since I have not configured any clocking on either routers as the Telco modems provide clocking. The only thing I noticed is that one router (1721) serial interface says Bandwidth 512kbit and later says available bandwidth 384 kilobits/sec in the show int s0 command, so I am wondering whether this has to do anythin although most problems happen during after hours .

I am attaching a show int s0 for the 1721 at our site.

Any help is appreciated

New Member

Re: Serial line problem

A couple of thoughts here.

1. Can you post the configs for each of the routers?

2. When the telco comes, have they ever looped the interface and performed a BERT test on the line?

3. How is this line terminated to you, via external CSU or internal Cisco CSU (and on each router)?

4. Has this always been this way, or has something changed?


New Member

Re: Serial line problem

1. I am attaching the two configs in a single file, the first is the 1721 at our site, second is the 2821 at the remote site.

2. The telco does a looped interface test to his exchange from our site. When the line was installed he used to get some errors but finally after 36 hours of testing the line was error free.

3. The line is terminated to an external modem which is connected to the routers serial interface via serial cable.

4. I have noticed drops in the line since it was installed, may be 2 - 3 times aweek for a couple of mins. but now it is very frequent and disrupts workflow.


New Member

Re: Serial line problem

I see nothing out of place in your configs; all appears well. How about a show interface on the 2821? Are you seeing the same errors on that end of the leased line? Additionally, what type of external CSU/DSU is used on each end and who did the configurations for each? Is the line encoding and framing properly set for both? Are any alarms going off on either end? How are those alarms reported?

Re: Serial line problem


Please note that you must NOT paste your config with passwords,since its not a live setup it wont do any harm, but pls.

To eliminate the problem pls do the following:---

(1)First test the serial interface of both the 2821 and 1721 as there isnt any problem.

To do this take one spare router and connect the WAN interface of both using DTE-DCE cable.

(2)Test both the V.35 cables if they are not the source of prob.

Now we know that the both the router and acc. are OK and we will move to Lease Line and modems.Your setup will be:

2821-->modem1(V.35)-->modem2(G.703)-->service provider<--modem3(G.703)<--modem4(V.35)<--1721

(3)I dont know which modem u are using,but lets see........Give loop from modem4 towards ur router.If u are getting the loop, without and errors/CRC then the connectivity between 1721 and modem4 is ok.

(4)Give Remote Loopback from modem4 and check the errors/CRC on 1721.If there are then conenct modem 3 and 4 back to back and see that there are no errors/CRC.If there are none then problem is in line,if not proceed further.

(5)Ask for a 4-wire hard loop from the telco towards ur 1721,if u are getting errors then there is prob. in modem 3.If not proceed next.

(6)Do the same for 2821 side, if no problem is detected then its definately TELCO side prob.Ask for a loop from modem3 side towards 2821 and see.

I hope above will give u the IDEA where the prob. lies when all the config. is ok



New Member

Re: Serial line problem

Thanks Paul, JD for your replies.

At each site the Telco provides preconfigured V.35 HDSL from Keymile (line runner). As I stated before the modem have been replaced at our site, and I have changed the serial cable.

The problem is that you will notice errors after a long time of activity, sometimes 2 days. The problem is temporary resolved after resetting the modem at our site.

The last time I reset the modem was sunday 8.20 a.m. and the line started flapping monday 5.26 p.m., again where there is no traffic because it is after duty hours.

I will try to test all cases as JD explained, and will ask the Telco to keep their devices for a while.

I will keep you guys updated.