Exam JN0-352 Topic 1 Question 152 Discussion

A GRE tunnel's outer, delivery-layer packets are forwarded using the router's ordinary underlay routing table, meaning the tunnel destination address (192.168.0.1 in this case) must itself be resolvable to a physical, non-tunnel next hop for the encapsulated packets to actually leave the router. Before this configuration change, the default route (or some more specific route) presumably pointed toward a real physical next hop, allowing the router to reach 192.168.0.1 and keep the tunnel operational. When the administrator commits a static default route of 0.0.0.0/0 with a next hop of gr-0/0/0, every destination lookup that previously fell back to the default route -- including the lookup for the tunnel's own destination address, 192.168.0.1, since no more specific route exists for it -- now recurses through the GRE interface itself. This creates a circular dependency: the router needs to route to 192.168.0.1 to keep the tunnel up, but the only route it has to reach 192.168.0.1 now points back into the tunnel that requires 192.168.0.1 to already be reachable, so the interface's next-hop resolution fails and the tunnel drops. This is an entirely valid and common GRE design pitfall rather than any platform restriction; GRE interfaces can be legitimately used as static route next hops, do not require loopback-sourced encapsulation, and do not require a /32 host route to resolve their destination, provided that route does not recursively point back through the tunnel.