I have two core routers for two campuses linked with a dark fiber. Each router is peering with a different ISP and each campus is advertising a seperate network but both networks use the same ASN. I was told that I will need to implement a GRE tunnel between tht two campuses and through the ISP's links in order for the two campuses to be able to talk to each if the direct fiber between them failed.
Isn't it possible that the two campuses use EBGP to talk to each other?
Note: Each campus border router receives a default route from its ISP, so there is no need for one campus to receive the route of the other campus.