sub-graphs for the route via the tunnel and the route for the tunnel's
destination are shown to the right and left respectively. The red line shows the
relationship form by stacking the two sub-graphs. The adjacency on the tunnel
-interface is termed a ԭid-chainՠthis it is now present in the middle of the
+interface is termed a 'mid-chain' this it is now present in the middle of the
graph/chain rather than its usual terminal location.
The mid-chain adjacency is contributed by the gre_tunnel_t , which also becomes