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How to configure isdn multilink connection with backup to second router?



I want to configure the following test scenario:

The router remote1 (Cisco 2611) of a remote location is connected to a central router central1 (7206) via isdn ppp multilink connection.

If central1 fails, remote1 should dial to central2 (a second 7206), also with ppp multilink.

OSPF is used as routing protocol.

It would be very fine if somebody could give me a sample configuration for this scenario.

Is it right that I have to configure the same ip subnet between the connection remote1<->central1 and remote1<->central2?

Further it should be prevented that remote1 dials 1 isdn channel to central1 and the second isdn channel to central2.

If you need further information please give me a short feedback.

Best regards,


8 Replies 8

paolo bevilacqua
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are the two central site PRI under the same telephone hunt-group, or do have different numbers?

This info is key to the configuration.


the two central site routers have seperate bri (or pri) interfaces for this connection with separate numbers.




With two separate numbers it's a little bit more complicated. You will have no other choice than configuring two dialer strings in the branch routers. It will attempt the first, then if it fails, will attempt the second one. You can configure the branch routers in two sets with the order of the strings inverted,to have some load balance.

There is a minor chance that the PPP multilink will terminate calls on the two central routers, however I would personally live with that rather than adding the complexity of configuring multichassis multilink, that is not well suited for enterprise networks.

On the central router the interfaces will be in the same subnet but with different IP addresses. Then you will configure floating static routes, and define all routing traffic as non interesting to the effect of keeping up the link. This will cause that once the primary circuit comes back, the ISDN link will be torn down.

Generally for simplicity and less overhead, RIp routing with filters is configured on hub and spoke networks. That does not prevent you from running OSPF in the core,with the appropriate redistribution.

Hope this helps, please rate post if it does!

Hi Paolo,

thanks for your answer, which nearly completely solved my problem.

Is it correct that both central routers and the branch router are connected to the same ip subnet?

E.g. the subnet is

central router 1:

central router 2:

branch router:

configuration of the branch router loooking like:


interface BRI0/0

no ip address

encapsulation ppp

no ip mroute-cache

dialer pool-member 1

isdn switch-type basic-net3


interface Dialer1

bandwidth 128

ip address

encapsulation ppp

no ip mroute-cache

dialer pool 1

dialer string

dialer string

dialer hold-queue 100

dialer load-threshold 1 either

dialer-group 1

no cdp enable

ppp authentication chap

ppp multilink




Yes. What are you missing is the dialer-list, that define which traffic is condired interesting in order to keep up the link. For this you specify an access-list that only permits, for example, unicast source from the branch LAn and nothing else. See:

Hope this helps, please rate post if it does!


I already configured it but didn't post it.

I will test this design and give a short feedback about the practical result.





why u r using complicate conf

use "backup" command

u can get all the docoment in cisco site


The problem with 'backup' command is thats it doesn't make the line go active when for example there is a 'black hole' in the network, or the FR PVC status is not reliable, or many other situations that dictates the use of floating static routes for a professional design.

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