Point to point T1 CRC errors

It could be not from your side but from telcos end.

Ask your telcos to check the link, they can do the loopback test from their site to your site.

Just put your link on loopback and ask them to test the line.

They will tell you the correct procedure.

Knight;

The telco usually supplies clock unless specified otherwise, I would check that and make a loopback plug to test against.  The clocking is going to be an issue you will get clock slip everyonce in a while if there are 2 clocks on the line which would make the circuit work but poorly... what is the distiance between the 2 sites?

Thanks
Scott

-Scott
Clock is not provided by telco. the physical distance between the sites is around 10-15 miles. Can you tell me if I have to provide clock on both CSU/DSU  or only on one?

Thank you  

there should only be one clock on a circuit .... the CSU should provide the Clock without a problem.  I would then create a lookback plug and run traffic to it see if you still get CRC's if not then it is the Telco

a loop plug is pin 1 crossed to pin 4 and pin 2 crossed to pin 5 and then ping something on the far side you won't get a reasponce but you will get errors if any of your hardware or cabling is bad.

Thanks
Scott

Your clock settings should only be provided by one side, the telco can provide it and while that may be more common it certainly isn't always that way.  If they don't provide it then one or the other side of the connection should provide it exactly as you have it setup.  If the clock settings were not correct; however, you wouldn't see CRC errors I don't believe, but rather you wouldn't be able to establish a connection at all and the line protocol on your router would show up as being down.

There are a lot of different causes for CRC errors, including a problem on the line from the Telco, but also duplex mismatches on the router or something like that.

Livedrive77,

You can still connect with 2 clocks I have seen it done.... there are problems when it happens.  I have also seen it were it will connect with no clock work for a short time and then the line will drop.

Knight
how many carrier transitions do you have in the serial interface?

Thanks
Scott

Scotty, I'll defer to your opinion on that then.  Anytime I've had to bring up new Ts I've always seen the behavior described in my comment, but that is certainly not an exhaustive cross section of experience...

Hi Livedrive

I am sorry but how do I check this.

I am sorry I mean Scott

You look into the router and if it is serial 0 you would do a show int s0 or show int s0/x or however it is described, it will be one of the very last stats that the display kicks out.

Thanks
Scott

does the line stay up for long periods then drop once and come right back up?  if so this is common of clock slip.

-scott

I just cleard the counters. I have 3 crc 0 frame and 0 carrier trans on local router. And 2 crc 3 frame and 0 carrier on remote router.

yes the line stays up for a while and then drops and again comes up.

Get the teclo involved ASAP that is what you pay them for ...

Now I have 20 crc 47 frame and 2 carrier transition on remote and 2 crc 0 frame and 0 carier on local

I have the telco doing the tests and I have confirmed with them that the clock is not provided by them.

what about the cabling from the CSU to the DMARC have you put a loopback plug into the CSU and seen if the errors continue.  At the rate your seeing them it would not take long to tell.

Thanks
scott

I don't know how to do a loopback. There is a loopbk button on the csu/dsu. The CRC on local are not increasing now but they are increasing on the remote router along with the frame errors. How do I do a loopbk on remote router?

you have to be at the remote side or have a CSU that is remotely managed....  To make a hard loopback take a RJ45 plug and one pair of cable from standard ethernet put one end into pin 1 and 2 the other end of the cable reverse the pin 1 to pin 4 and reverse the pin 2 to pin 5.

Thanks
Scott

http://www.ossmann.com/5-in-1.html check this out for a loopback plug picture.

Thanks
Scott

scott

I did the loopback button on the csu/dsu. I got no CRC but got 20 carrier transitions.

did you run traffic to it?  When you did a show int s0/x did it say up and up (looped)

thanks
scott

yes it said

line protocol is down (looped)

are you using HDLC or PPP as the line encapsualtion?  and this must be the side with no clock in the CSU right?

Thanks
Scott

and I just pingged the remote router

I am using PPP on both ends and yes this(local) side is the one with no clock

ok that would explain the line up protocol down.... so you ping the remote router with the CSU in loopback and get no errors but line transitions?  I would still recomend using a loopback plug in the CSU or in the TELCO DMARC to test with this eliminates the cabling between the CSU and the TELCO and puts the burden squarely on them.

Thanks
Scott

Scott
 I have a rj45 female jack. which pins should I punch?

Ok I have connected 1-4 2-5 and router is saying looped. I am pinging the remote router.

Scott

It is the same result. No CRC but Carrier transitions. I am going to the remote site to do similar tests. What do I do if I am getting no CRC in loopback and if I am getting errors in loopback.

Thank you

I don't think you will get any CRC errors with this loopback.

With this your are testing your router.

CRC problem occurs due to problems in the last mile connections(mostly).

You need to ask your telcos to put your link on loop from his side.

Then, you need to check it for CRC errors.

This is how a link is given to you by telcos.

Router--------------------------Telco------------------------------------Telco------------------------Router
Site-1                                   T1                                                 T2                                Site2

Now, you need to check the connection betwenn T1-Site1 and T2-Site2.

Telco to Telco will never cause the CRC errors.

With the loopback what you are trying to prove is that the CSU and all the cabling up to the TELCO DMARC is good that is all you want to accomplish if you can do that then it is up to the TELCO to fix the problem.

Thanks
scott

Thank you guys. I am going to the remote site for testing and will be back again:)

Thank you

Ok knightrider2k2
Please Check Earthing at both ends, I think the power supply which you are provinding to both router doesn't have proper earthing thats why you are receiving so many CRC error, correct your earthing then clear the counter on routers & monitor it. Your CRC errors will not come then.

Hi Guys

It was clocking. According to Cisco

"If you are attempting serial connections at speeds greater than 64 kbps with a CSU/DSU that does not support SCTE, you might have to invert the transmit clock on the router. Inverting the transmit clock compensates for phase shifts between the data and clock signals."

Thanks for all your help.

YES!!! glad it is fixed .... I have some experiance with the same issue and the way we used to mux a 64K channel so that is why I was pretty sure that was causing this issue.

Thanks
scott