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New Member

ISDN Backup Anomalies

Hello all,

I was hping that someone could help with this rather unusual problem I have experienced at a client site.

The client has two sites connected with a T1 and has ISDN BRI lines for backup. Each end has a 3640 router. One end has a T1/CSU/DSU/FE NM, 4 port BRI and 4 port Serial (AS/400). The other end has a 1 FE NM with a T1 and a BRI WIC, a 4 port Serial (AS/400 and a 4 port Ethernet. IOS on both usits is 12.0.4 - IP/IPX. OSPF is being use as the routing protocol. IP and IPX are both being routed. The AS/400 traffic is STUN encoded and passes from loopbak to loopback.

Here's the problem.

The T1 failed and the ISDN Dialer interface established the call. The AS/400 STUN traffic was able to cross the link, but no other IP or IPX traffic could cross the link from either side. Traceroutes would fail at the local router every time.

The IP routing tables appeared to be correct on both sides, and the routers could talk to each other and could telnet from one to the other. Did not check the IPX routing tables.

The stangest thing I saw afer trying to simplify things, etc was that I could sometimes get a single ping to cross the link and then nothing else.

I don't have to appropriate scripts onhand at this time, but they were based upon "cookbook" scripts from TAC and when the routers were deployed, 2+ years ago, everything was supposedly tested.

Any quick ideas?

I'll probably post more detailed info when I get the laptop hooked up over the weekend.

We are scheduling some downtime on Monday to resolve the ISDN issues.


Bob Slimmer

New Member

Re: ISDN Backup Anomalies


You already have the hardest part functioning- the call is being made and answered. Look at the backup interface. IPCP and IPXCP should be open when a call is up. Is OSPF forming an adjacency across the backup circuit? If so, you should have OSPF routes. If not, make sure you have the things setup as needed to form an adjacency. On the backup circuit; the same OSPF area, same MTU, same OSPF network type. And the OSPF aea has to be connected to the backbone area.

It's odd that the STUN traffic was OK. Are you using IP UNNUMBERED LO0 on the backup links. If so, it's usually easier to trouble shoot using real IP addresses on the backup circuit.



New Member

Re: ISDN Backup Anomalies


First off, thanks for replying. Based upon the info in your message, it would appear that the issue is with OSPF forming an adjacency. I did note that when the backup link was operating, the remote router was NOT showing as an OSPF neighbor. I'll try adding the following to the Dialer interface

ip ospf network point-to-point

ip ospf demand-circuit

I'll let you know if it works.