Attached is the basic diagram of our scenario here. EIGRP is fully deployed throughout the network. If the link between corpcoresw01 and XO Router-Dallas goes down, it will bring up a VPN between the 2 firewalls (Cisco 5520's- IPSEC). Once the VPN comes up- how will corpcoresw01 and dalcoreswp01 share their routes? Does a GRE tunnel need to be established to bring up the peering relationship? What is the best practice way to accomplish this?
A GRE tunnel is a nice straight forward way of doing it. Make sure you don't permit recursive routing to happen. Make sure the tunnel endpoints are forced to only run via the IPSec tunnel (often easiest to use permanent static routes).