T1 PPP Multilink and LCP Negotiation

Answered Question
Jul 19th, 2007

We have a typical T1 bundle via PPP Multilink. We bundle two T1s together and we do this seventeen times over to our local offices and it works great on most links. However, a couple are problem links where only one T1 is active in the bundle. As for the other T1, I beieve what is happening is that the LCP is trying to negotiate to an "LCP Open, multilink Open" state to enable multilink on the port. LCP is constantly changing between the following states:

LCP Listen, multilink Closed

LCP TERMsent, multilink Closed

LCP ACKrcvd, multilink Closed

I am wondering if anyone knows if this problem may be due to a non-typical telco device somewhere in the line? And also if there is a way to statically configure the LCP state. Any help would be great, thanks!

I have this problem too.
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Correct Answer by bbaillie about 9 years 4 months ago

Multilink needs to identify how to differentiate between bundles,

this said if you use more than one multilink on a router you need to identify the member links of the bundle. When "not" using authentication the method is to use an endpoint dicriminator for the serial

links to facilitate the bundle identification by the local router.

Endpoint discriminators are locally specific to the router so if each serial port is assigned to an individual multilink bundle then use unique endpoint discriminator values for each serial link, no need to make them match on source and target router. If more

than one interface is assigned to the same multilink bundle they must have the same endpoint discriminator value. Long winded but here is an example.

interface Serial 0/0

encapsulation ppp

ppp multilink

multilink-group 122

ppp multilink endpoint string multi122

interface Serial 0/1

encapsulation ppp

ppp multilink

multilink-group 122

ppp multilink endpoint string multi122

The endpoint discriminators can be arbitrary strings but for ease of identification

use something that resembles the multilink group identifier.

When each port is assigned to an individual multilink bundle the discriminator

values need to be different, in your case they must match.

Cheers,

Brian

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Richard Burts Thu, 07/19/2007 - 08:49

Aaron

I have not seen this symptom before but hope that it would not be difficult to troubleshoot and resolve.

First I do not think that there is any way to configure the LCP state. LCP negotiates so that it can dynamically learn and verify the state. If there really were a problem and you had managed to hard code the state to look good then that would be bad.

I would first look carefully to be sure that the routers and interfaces on each end matched up and had exactly the same configuration parameters. It might be helpful to take the T1s out of the multilink bundle and run CDP over them to verify that everything is connected the way that you think.

If all that looks good I would suggest running debug and looking at the PPP negotiation on the link. Hopefully that will point to the problem.

HTH

Rick

aaronsj Thu, 07/19/2007 - 17:34

Rick,

Thanks for your post. I actually was able to pinpoint the problem. Apparently, when the Multilink interface was initally configured, it grabbed the distant router's hostname for the multilink name. Recently, we changed hostnames on the routers to adhere to a new router naming convention. This threw off the multilink as it had the old name in memory. I simply removed the serial interfaces from the bundle, added them back and the name adjusted to the new router hostname. Is there a way to statically name the multilink bundle (not a description), but the multlink itself so that I don't have this issue in the future?

Correct Answer
bbaillie Fri, 07/20/2007 - 02:18

Multilink needs to identify how to differentiate between bundles,

this said if you use more than one multilink on a router you need to identify the member links of the bundle. When "not" using authentication the method is to use an endpoint dicriminator for the serial

links to facilitate the bundle identification by the local router.

Endpoint discriminators are locally specific to the router so if each serial port is assigned to an individual multilink bundle then use unique endpoint discriminator values for each serial link, no need to make them match on source and target router. If more

than one interface is assigned to the same multilink bundle they must have the same endpoint discriminator value. Long winded but here is an example.

interface Serial 0/0

encapsulation ppp

ppp multilink

multilink-group 122

ppp multilink endpoint string multi122

interface Serial 0/1

encapsulation ppp

ppp multilink

multilink-group 122

ppp multilink endpoint string multi122

The endpoint discriminators can be arbitrary strings but for ease of identification

use something that resembles the multilink group identifier.

When each port is assigned to an individual multilink bundle the discriminator

values need to be different, in your case they must match.

Cheers,

Brian

aaronsj Fri, 07/20/2007 - 15:31

Brian,

Thank you for this info. I will probably do this sometime down the road, we aren't changing hostnames again for a while (or forever hopefully!) but I think it worthwile to do as good preventive maintenance. Thanks again.

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