ISDN problem

[josephqiu]

I have two routers (2611 and 804) connected by ISDN line. Today for some reason, the ISDN line dropped off and it came back after a few minutes. However, only one B channel is up, and the other one is still down. Used "show isdn status" and found only "1 Active layer 3 call(s)".

I'm not so familiar with ISDN. Could someone tell me what might be the reason? How can I get both B channels up?

Thanks a lot!

[Georg Pauwen]

Hello,

are you talking about two connections, of which only one succeeds, or are you talking about one connection, which only uses one B channel ? In the latter case, make sure you have ppp multilink enabled on both sides (with the interface command ´ppp multilink´), otherwise the second channel will not be used. The threshold for activating can be configured with the ´dialer load-threshold X´ command, where X is a value between 1 and 255. Let´s say you want the second B channel to be activated when the load of the first channel exceeds 50%, the dialer load-threshold would be 127.

You can also automatically enable both B channels with the interface command ´ppp multilink links minimum 2´, which would bring up both B channels right away, regardless of the load of the first B channel.

If you use dialer interfaces, make sure you have the ´ppp multilink´ statement configured on the BRI physical interface as well, in addition to the dialer interface.

If you second B channel won´t come up regardless, there might be another problem. Have you tried rebooting the routers ?

HTH,

GP

[josephqiu]

Thanks for your reply.

First, I'm talking about one ISDN link with two B channels.

I actually have both "ppp multilink" and "dialer load-threshold" configured. X is set to 2 in my case. I never had this problem before.

I tried to use "ppp multilink links minimum 2" command, but my router doesn't like this command.

So I had to reload the 804 router at remote site. After reload, one of the B channels came up immediately. The 2nd one came up after about 5 minutes. They look good now. But I still don't understand why.

Anyway, thanks a lot for your suggestions!

[Kevin Dorrell]

The second channel comes up only when there is enough traffic to need it.

From the way you are talking about the ISDN, you may be considering it as a link between two sites. It is not; it is (normally) a dialup service. Each B-channel is an independent dialup service.

ISDN was originally conceived for digital telephones and dialup accesses, used like a modem, but digital. It can handle two connections at the same time, hence two B-channels. Normally, you are paying for each call seperately. It just so happens that you are dialling both calls to the same site so you can use the links in parallel and get more bandwidth out of it. That is why you don't open the second call unless you need it - if you open two channels, you pay for two calls. You are paying for each call by the minute.

Now to multilink - it is not necessary to configure multilink, even if you have two channels open to the same destination, but it does make the traffic distribution smoother. If you have two channels open to the same destination, but no multilink, then the traffic will be shared between the two links according to the source and destination IP addrsses. So if you have two machines that are generating most of the traffic, then that "most of the traffic" will go down one single link. This configuration works fine if you have lots of conversations going on, and they are evenly balanced. On the other hand, if you have multilink, each packet gets split up, and shared over both links. This results in a more even distribution of the traffic between the two links, but does have some protocol and processing overhead.

Hope this clarifies things.

Kevin Dorrell

Luxembourg

[josephqiu]

That's very helpful information. Thanks a lot!