Configuring ISDN backup with two PRI connections

Unanswered Question
Mar 10th, 2009
User Badges:


I have two main sites, every with router with ISDN PRI link. Other branch offices have ISDN BRI link for backup and they will connect to both sites.

I'll use watch groups on branch side and two dialer string command. If first main PRI link doesn't work, it tries to dial the second one.

But, I have doubt how to configure main sites, how to configure dialer interfaces (can I use only one dialer interface) and what is with IP address, should I use unnumbered interfaces or put static IP addresses for every branch office? How will this work.

Does anyone have suggestions how to configure these routers?

I don't have radius or tacacs server and all configuration shoul be on routers.


  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Giuseppe Larosa Wed, 03/11/2009 - 03:02
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Vlaho,


you can use a single IP subnet per PRI and so you can use a single dialer on the core router.

I would use two dialer interfaces on each remote with dialer map command

two floating default routes with one with lower AD

the suggestion is to use the dialer map command that allows to associate remote hostname, remote ip address, remote isdn number in a single command.

This helps also for authentication

you need then a username/pwd pair for each remote

about routing in this way you can configure floating static routes that uses directly the ip next hop of remote sites.

You then need to redistribute the floating static routes in your IGP

Verify both the failure and the restore events: the backup routes have to be removed from IGP when primary link comes back


you can build a hierarchy of backup routes by using different seed metrics in redistributing floating static routes on the two core routers

Hope to help



This Discussion