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I've trawled the Internet for hours on this and have found nothing but contradicting Cisco documentation.

I have, on a single router, fa0/0 which is connected to a VDSL2 line and dials this with Dialer0. I also have, on this router, atm0/1/0 which is connected to an ADSL line and dials this with Dialer1.

When each is dialed, they get assigned the same IP address, gateway, etc. A routed subnet is passed over these.

When the ADSL line is dialed, the ISP starts sending all incoming traffic down it. I can, however, force all outgoing traffic down the VDSL2 line... it's just that replies come down the ADSL.

How is best to configure this such that the ADSL line is used in the event of a failure on the primary VDSL2 line? I know about ip sla and floating routes with track. My assumption is that I need a dial on demand config on the ATM interface or Dialer1 interface to only dial this when traffic is being forced out of it with the floating route.

Any pointers would be much appreciated.

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Do you have a way to detect a failure on the vdsl link? – petrus Mar 20 '13 at 20:34
I'd suggest probably by testing for reachability of the next hop gateway over it. I appreciate that this isn't fool proof as a failure can be considered as a lot of different things. – phil-lavin Mar 20 '13 at 23:23

Do you know about Cisco's backup interface feature? The Cisco docs are a bit flaky on it, so I here is a link to a blog where I believe the author is explaining it reasonably well.

You can try this on the dialer interface for atm0/1/0.

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