Cisco Support Community
Community Member

ISDN Routing !!!

Hi any one please help to clarify the following scenerio;

I have a remote site with 1720 router and connected back to HQ with a 2620 router there by a 64k leased line. There is a 64k ISDN backup link for the two sites.

The BRI interface is belonged to a member of a dialer profile. Once the primary link (64K leased line) fails, the ISDN will perform a DDR as a backup.

If I use the following static default route for the primary link;

1720(config)# ip route

whereby is the ip address of the Serial interface of 2620 at HQ.


(1) what should include the routing for the ISDN link ?

(2) How do I differentiate the routings for Leased line and ISDN link ?

(3) At HQ do I need to specify "ip route" (whereby is the ip address of 1720 serial interface) ?

(4) If I have more than one remote site connected to HQ, can I still use the routing as in (3) ?

Please help out.


Re: ISDN Routing !!!


add another route on the 1720, with the ISDN interface as next hop, with an administrative distance of 2.

So you will have two static routes,

ip route serial0

ip route bri 0 2

By default static routes have administrative distance of 1. The second default route will have admin distance of 2. The second route will kick in only when serial0 is down. When serial0 comes back up, the second route will go off due to higher administrative distance.

You can use same config at all remote sites.

Community Member

Re: ISDN Routing !!!


I suggest you have a look at : where floating static routes are explained.

Also a good configuration example is given and explained.

Basicly :

You set the route through the LL with an admin distance of p.e. 1 (as you did as it is the default for static routes).

You set the route through the ISDN with a higher administrative distance (p.e. 200).

If the router finds no route through an active interface using the 1 admin distance he will look for a route with a higher admin distance, and so that route (in your case the ISDN) will float to the surface and the router will start using it, until the LL comes up again.

If you need more help, please don't hesitate to contact me.

Frank Hofman


Re: ISDN Routing !!!

If you are trying to use static routes then you need two static default routes at the remote end (1720). One like you have and one pointing to the other end of the ISDN link that is weighted so it will be ignored as long as the leased line is up. Something like x.x.x.x 200 where x.x.x.x is the other end of the ISDN.

The routes at the host end need to point to the network on the other side (ethernet) of the 1720, not a default route, unless these are two isolated, stand alone networks. There should be two an they should be weighted the same as the ones in the 1720 so the ISDN route will be ignored.

To have multiple remotes the host will use dialer map statements.

CreatePlease to create content