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I also assume that you must have network reachability between the tunnel source and the tunnel destination and vice versa.
Yes and nothing blocking the tunnel packets.
BTW, on GRE tunnel, the tunnel can appear up/up when it's not. Pinging the other side of the tunnel should confirm tunnel is fully operational. (NB: for GRE tunnels, using keepalives will take the tunnel down if the other side can't actually be reached.)
