Rick,

I totally agree with you that BGP would be required if his routers are receiving the full/partial internet routing table and using the AS-path info to route to the internet

But If the user is using BGP only to advertise his subnets and just uses a default route for routing to the internet then an IGP can be used between his routers. (for eg ospf can be configured between his routers to send the default information)

Narayan

Narayan, only saw your post after my post. I too had thought about usage of default routes and an IGP, but since the OP asked about "in case one of the ebgp path is inaccessibe", I think we need to assume we're working with full route tables. Otherwise a default could forward a packet to a next hop that has the missing path. (Of course, why one ISP has a path/router that the other doesn't, is a different issue.)

[edit]

Re-reading the OP, if we're talking about the physical path is lost to the one ISP, then the default route might save the need to pass one router's routes to the other, which could be done via an IGP, although I still wonder whether is would be advisable not to have the two internal routers in BGP communication (even if they filter their routes and pass a default).

[edit2]

If I'm not clear, I read the original post as "in case one of the ebgp path is inaccessibe", meaning one external path (route) is lost from one ISP, but Narayan's implied interpretation, that the path on one ISP is loss, I now think is more likely correct.