.I have wireless guest always take the path of ADSL which i can do via routing-instance type forwarding instance .
But while users behind Core Switch using MPLS link to connect different sites and access centralize internet.Once MPLs goes down then i need to switch to them ADSL to initialize VPN to central site to make it work
Below are the brief point to make it more understand
-->Wireless Guest use ADSL
--->Corporate users use MPLS
-->Once MPLS goes down VPN initate through ADSL for Corporate users