We are currently using a private DSL network provider as a primary network connection and ISDN dialup as backup. The central office has a Cisco 3640 router, and the remote locations have a mix of Cisco and Netopia depending on the local network providers and the type of DSL. Note that the DSL connections from the remote sites all pass through the network provider and come in to the central office on a single network connection, but the ISDN backup dials directly from a remote site to the central office. So far, this has worked well with each remote site router making the decision to dial ISDN backup if its DSL connection goes down, and the remote routers all send RIP to the central 3640 router so it knows which interface to route back to for each remote site.
We are now trying to configure a new remote site with an ADSL connection to the network provider. The way SBC has the DSL set up, a Netopia 3366C-ENT router can properly handle dial backup but cannot send RIP, and a Cisco 837 router can send RIP but cannot do dial backup. Without RIP, even if a remote site is dialed in, the 3640 tries to route over the primary WAN ethernet as the signal will always be there, and that interface is shared with all the other sites. We considered making a static route to the ISDN dialer in the 3640 with a lower administrative distance than the static route for the primary WAN, but then the 3640 will just sit and wait for the remote site to dial in as the dialer is always there even if an active connection is not.
Is there another way for the Cisco 3640 on the receiving end to know that a remote site has dialed in on the ISDN and route its subnet to that interface, but use the default WAN ethernet if it does not have a dial-up connection?