Interesting DCI Problem

[Otanx]

There is also OTV, L2TPv3, and some others if you really need to span L2. I try not to do it so I don't know the best way. Are you NATing this address space on the firewalls to servers inside? Can you move the firewalls external addresses to different space, and then have them each advertise the /24 into OSPF with different metrics? Then you set the L3 link between data centers to be larger than the difference of the advertisements. That way local DC traffic stays local, but in a failure they will still work.

-Otanx