There are several geographically separated intranet network access servers that route to each other via GRE Tunnels. Each NAS has the ability to static Dial-on-Demand route to several of the same extranet NASs via asynchronous analog modem connections. Each extranet NAS can only accept a single dial-in session. How can the intranet NASs be configured to route to one of it's peers' interfaces with an existing dial-out connection before attempting to dial-out through one of its own interfaces?
Note: Ideally no broadcast routing protocol would be necessary on the extranet NASs.