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New Member

Backup broadband with ISDN?

Does anyone have any real world experience of backing up a broadband connection with ISDN?

I propose to connect two sites via an ADSL service.

The VPN will be implemented on Cisco1720 with VPN software and hardware. The Telco will provide static IP's and ADSL to ethernet router. My router will therefore have dual ethernet. I would like to use ISDN to provide a backup to this link, and envisage one router dialing the other directly in the event of either site loosing its ADSL. The ISDN backup could be on another router if this makes it easier (HSRP).

Backup ISDN for serial links is easy since you loose the clock to the other site and can switch in the backup link. Later, the clock returns, the interface comes up and the backup is switched out.

With ADSL this is not possible, since the router just sees a permanent connection to the Telco's router.

I think maybe I need floating static routes, or OSPF hello's.

Anyone done this in anger?


Cisco Employee

Re: Backup broadband with ISDN?

You can use the floating static route OR the "dialer-watch" command.. With that router will watch for the defined route, once its not there in the routing table, it will activate the backup link..

Pl. visit the following url for more information

For configuration and troubleshooting:

For all 3 ways :

For dialer-watch:

Hope it helps..Thx..Tejal

New Member

Re: Backup broadband with ISDN?

Thanks Tejal

I will have a look at Dialer-watch. Does this not depend on the learning speed of the routing protocol in use? Which ones are best for the job?

Of course the next question is - how does the customer know that he is running on ISDN (and running up a large phone bill....?)

New Member

Re: Backup broadband with ISDN?

Hi all,

Along these same lines I need to do dial backup as well. My problem is I have a Primary VPN connection using ADSL and want to create a secondary VPN connection with a US Robotics modem hanging off my AUX port. When the tunnel goes down on the Primary, I need it to initiate dial backup on the AUX port, create a VPN tunnel and Terminate to the same VPN head end that the primary tunnel was terminated to.

Can this be done?


Jerry Roy


New Member

Re: Backup broadband with ISDN?


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