Routes can be used to forward traffic only when they have directly connected next hops. However, this condition may not be met when routes are generated. Therefore, the system needs to search for the directly connected next hops and corresponding outbound interfaces, and this process is called route recursion. In most cases, BGP routes, static routes, and UNRs do not have directly connected next hops, and route recursion is required.
For example, the next hop IP address of a BGP route is the IP address of a non-directly connected peer's loopback interface, and therefore, the BGP route needs to perform recursion. Specifically, the system searches the IP routing table for a direct route (IGP route in most cases) that is destined for the next hop IP address of the BGP route and then adds the next hop IP address and outbound interface of the IGP route to the IP routing table to generate a FIB entry.
The next hop IP address of a BGP VPN route is the IP address of a non-directly connected PE's loopback interface, and the BGP route needs to recurse to a tunnel. Specifically, the system searches the tunnel list for a tunnel that is destined for this loopback IP address and then adds the tunnel information to the routing table to generate a FIB entry.