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ISDN goes up and down


I have ISDN PRI configured as a backup line, with DDR Dialer Profiles.

I checked the Admin Distance values: Main routes are OSPF-learned (110), floating route through ISDN (200).

Despite of the preferred OSPF Routes, there one host that's still opening the Dialer interface.

If I use an ACL against that host, it would prevent it from accessing the WAN at all.

What can I do to change this up-and-down behaviour?

I attached the Dialer1 config and a Syslog output.


Re: ISDN goes up and down

If the state change was unexpected ,confirm the configuration settings of the interface.

See the below URL which explains about ISDN configuration:

Cisco Employee

Re: ISDN goes up and down


I didn't find a complete conf. with your dialer ACL. Basically run debug isdn q931 and debug dialer and see whats trigger your isdn line.

If it's dialing all the time it will be easy to catch. If its random just leave the debugs on and see whats triggers it. Also have you had any issue with your isdn line staying up when it suppose to be up ? I noticed your idle time out is only 20 seconds. Its been my experiences you should have it over 30 seconds as some time 20 seconds is not enough for ppp to fully establish. 35 sec is fine but 20 seconds ive seen issues but, that's just a side note.

Thx Steve

Hall of Fame Super Silver

Re: ISDN goes up and down

Hello Wassim,

you need to verify what IP address that IP host is trying to contact.

Verify if you have a default route in OSPF.

If a packet for an unknown subnet is received and no OSPF route is available this will trigger a DDR call.

I think you have a floating static route via dialer interface :

ip route DialerX 200

but if net is missing in OSPF DDR is triggered.

Hope to help


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