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Backing-up Frame Relay with ISDN

Is it possible to configure a dial backup interface (ISDN) to inherit the IP Address of the WAN Interface (Frame Relay) which it backs up?

The question stems from a situation wherein a network has existing IBM 2210 Routers connected via Frame Relay to an IBM 2216 router at the central site. If any of these branch routers loose connection via Frame Relay, it would dial to another IBM2210 Router at the central site using ISDN. this link will inherit the IP address of the Frame Relay link that has gone down.

We are now adding new Cisco 2620 routers for new branches. The primary frame relay links are working fine. ISDN backup needs to be configured in the same manner as the existing routers. Is it doable?


Re: Backing-up Frame Relay with ISDN

No aware of any such feature where the ip address is inherted off a down interface, the problem will arise when the primary link comes up & we have both the links (primary & backup) up for a while together until the backup is torndown (say after expiration of idle timer), so we will need different ip addresses on the router on the 2 different interfaces.....using the backup command & backup delay under the frame link with ip unnumber (to say loopback interface) under the frame & bri might just work...frankly have never tried the same.

Thanks, Mak.

Cisco Employee

Re: Backing-up Frame Relay with ISDN

You can use the advantage of the command "ip unnumbered ". That way you can create the loopback 0 interface and assign ip addres which you want that to be assigned to wan interface and isdn interface. After that you can use both the wan and isdn interface with "ip unnumbered loopback 0" command.