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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.

2 Replies 2


You need a floating static route for the ISDN interface; something like "ip route 200", where is the 2620 ISDN address. If you are using unnumbered on the ISDN interface, then you also need to add "ip route dialer 1", where Dialer1 is your dialer profile. Once the serial link goes down and is removed from the routing table, this new default route will be installed.

At HQ, for normal routing, you need to have a route for the LAN (or other networks) of the 1720; so something like "ip route". You would do this for each remote site.

You could also use RIP routing over ISDN to inject a summary of the branch into HQ, with an appropriate admin distance.

Once the serial line comes back up the EIGRP (or OSPF?) reconvergence should replace both the RIP injected route at HQ and suppress the floating static in the remote office.

RIP is handy in this case because the normal routes become valid while ISDN is still up so there's less likelyhood of loss of session problems when it gets torn down, if you see what I mean.

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