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Three Cisco routers form the lab, with the middle router carrying the underlay traffic.
Verify that the far tunnel endpoint is reachable through the physical network.
Configure a GRE tunnel on each edge router using its loopback as the source.
Check that the tunnel is up and that the overlay route uses it.
Ping the far tunnel address to verify that traffic crosses the overlay.
Shut down the far side of the transit router to break the underlay path.
Both the endpoint and the overlay become unreachable.
Restore the transit link and verify that the endpoint and overlay are reachable again.