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BRI backup for ADSL

I am looking for sample configurations for 1700 series router with...

ADSL Internet access

BRI backup (router-to-router)


Have implemented the ADSL access and VPN but a bit in the dark regarding ISDN backup. Someone suggested I look at GRE and a routing protocol.

Some sample configs would be helpful as I need to add the ISDN backup on a live network. Don't want to mess up.


Re: BRI backup for ADSL

The router generally uses one of these three procedures to monitor the primary connection and initiate the backup connection when needed.

Backup Interface

Floating Static Route

Dialer Watches

You would have to evaluate these and decide which best suits your requirement and implement it accordingly.

Use the information in this document to help your decision.

You can also refer to the following samples for configuration.


Re: BRI backup for ADSL

The person who suggested the GRE tunnel to support a routing protocol had the right idea. The challenge with a VPN tunnel is to detect when it is down so that an alternate route can be selected. The ADSL interface almost never goes down, so classical dial backup approaches such as backup interface won't work. You must use an approach which tracks availability of the other side based on a routing protocol or "outside information." Cisco does not support the latter (but look at Nexland Pro800 Turbo or Symantec 200R for an example of how it can be done).

Using Cisco kit, your only choice is running a routing protocol, which requires a router at each end of the VPN. You can use BGP directly over the VPN, or any interior routing protocol (EIGRP, OSPF, RIP, etc) over a GRE tunnel. Last time I looked, only the GRE tunnel approach is documented here on CCO, but there is a white paper with an example of each approach on my web site.

Good luck and have fun!

Vincent C Jones

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