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ISDN BRI idle-timeout problem [BRI as a backup interface]

Hello to everybody,

Let me explain the problem:

On a Cisco 2600 router, "Interface Serial1/1" is the primary link to HQ. BRI0/0 is configured as a backup interface of Ser1/1. EIGRP is the dynamic routing protocol in between the brach and HQ. BRI initializes itself in just 10 seconds after Ser1/1 goes down. Upto now, everything seems o.k. However, ISDN link goes down/and gets up immediately within 120 seconds after the BRI interface is comming up. And whole scenerio repeats itself in every 120 sec period.

It is not an expected action indeed. Because dialer idle-timeout equals to 600 sec, and all ip traffic is interesting (please check dialer-list 1). Besides, EIGRP hellos should make the ISDN link up until the primary link goes up again, shouldn't it?

I could not conclude in a meaningful reason for this problem. IOS version of the branch router is 12.1(4) IP Plus feature set [c2600-is-mz.121-4.bin]. I have a doubt on IOS bug.

Relevant config can be seen below. Any help will be useful.



interface BRI0/0

ip unnumbered Loopback0

encapsulation ppp

dialer idle-timeout 600

dialer string 902123501900

dialer load-threshold 180 either

dialer-group 1

isdn switch-type basic-net3

no cdp enable

ppp authentication chap

ppp multilink



interface Serial1/1

backup delay 10 15

backup interface BRI0/0

ip unnumbered Loopback0

no ip mroute-cache

no fair-queue



router eigrp 1200


no auto-summary

no eigrp log-neighbor-changes


dialer-list 1 protocol ip permit



Re: ISDN BRI idle-timeout problem [BRI as a backup interface]

Is the remote side configured as a dialer interface. I have seen scenarios in which the dialer interface starts counting down from the hub side and brings the line down. If it is a dialer do a 'sh dialer' on the hub side and watch to see if the timer is counting down.

You may also want to look at the load on the bri, perhaps the circuit is being over utilized causing the line protocol to drop.


Re: ISDN BRI idle-timeout problem [BRI as a backup interface]

120 sec is the default idle timeout. This sounds like the router at the other end is not configured to consider any of the traffic it is sending to be "interesting." If that router is not configured to dial out, it is safe to tell it that any IP traffic is interesting.

On the other hand, if the other router is already configured to consider anything it sends to be interesting, you could have a routing problem that causes the ISDN line to not be used for outgoing traffic. While much rarer, I have seen it happen.

The key to find it is to log into both routers and do a "show dialer." One of the two will be seen to be counting down to zero in the idle-time field. That's where you need to look.

Good luck and good hunting!

Vincent C Jones

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