When a packet arrives on a router interface, the router examines the IPv4 header, identifies the destination IPv4 address, and proceeds through the router lookup process.
In Figure 1, the router examines level 1 network routes for the best match with the destination address of the IPv4 packet.
1. If the best match is a level 1 ultimate route, then this route is used to forward the packet.
2. If the best match is a level 1 parent route, proceed to the next step.
In Figure 2, the router examines child routes (the subnet routes) of the parent route for a best match.
3. If there is a match with a level 2 child route, that subnet is used to forward the packet.
4. If there is not a match with any of the level 2 child routes, proceed to the next step.
In Figure 3, the router continues searching level 1 supernet routes in the routing table for a match, including the default route, if there is one.
5. If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet.
6. If there is not a match with any route in the routing table, the router drops the packet.
Note: A route referencing only a next-hop IP address and not an exit interface must be resolved to a route with an exit interface. A recursive lookup is performed on the next-hop IP address until the route is resolved to an exit interface.