506 on prem - vASA in the datacenter - MPLS & VPN between the two and want to auto failover between the links - is that possible?
do you have two different interfaces, one for MPLS and another one for Outside?
I think that you should be able to do that using a policy-map on the inside interface.
I suppose that you can try to change the next-hop from outside to the MPLS interfaces and track this rule so in case of failure, all traffic should go over VPN.
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