Tunnels share a similar property to recursive routes in that after applying the tunnel encapsulation, a new packet must be forwarded, i.e. forwarding is recursive. However, as with recursive routes the tunnel's destination is known beforehand, so the second lookup can be avoided if the packet can follow the already constructed data-plane graph for the tunnel's destination. This process of joining to DP graphs together is termed stacking.
Figure 11: Tunnel control plane object diagram
Figure 11 shows the control plane object graph for a route via a tunnel. The two 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 'mid-chain' since 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 part of the FIB control-plane graph. Consequently it will be visited by a back-walk when the forwarding information for the tunnel's destination changes. This will trigger it to restack the mid-chain adjacency on the new load_balance_t contributed by the parent fib_entry_t.
If the back-walk indicates that there is no route to the tunnel's destination, or that the resolving route does not meet resolution constraints, then the tunnel can be marked as down, and fast convergence can be triggered in the same way as for physical interfaces (see section ...).
Multi-point tunnels are an example of a non-broadcast multi-access interface. In simple terms this means there are many peers on the link but it is not possible to broadcast a single message to all of them at once, and hence the usual peer discovery mechanism (as employed, e.g. by ARP) is not available. Although an ip_neighbor_t is a representation of an IP peer on a link, it is not valid in this context as it maps the peer's identity to its MAC address. For a tunnel peer it is required to map the peer's overlay address (the attached address, the one in the same subnet as the device) with the peer's underlay address (probably on the other side of the internet). In the P2P case where there is only one peer on the link, the peer's underlay address is the same as the tunnel's destination address. The data structure that represents the mapping of the peer's overlay with underlay address is an entry in the Tunnel Endpoint Information Base (TEIB); the tieb_entry_t. TEIB entries are created by the control plane (e.g. NHRP (RFC2332)).
Each mid-chain adjacency on a multi-point tunnel is stacked on the fib_entry_t object that resolves the peer's underlay address. The glean adjacency on the tunnel resolves via a drop, since broadcasts are not possible. A multicast adjacency on a multi-point tunnel is currently a work in progress.