A device performs a route lookup for learned BGP4 routes to find the IPv4 address of the next hop. If the BGP4 next hop is not the immediate next hop, a second lookup (a recursive lookup) is required to resolve the exit path for destination traffic.
A BGP4 route is eligible for addition to the IPv4 route table if the following conditions are true:
By default, a device performs only one lookup for the next-hop IPv4 address. If the lookup does not find a valid next hop IPv4 address, or if the path to the next hop IPv4 address is a BGP4 path, the BGP4 route destination is considered unreachable. The route is not eligible to be added to the IPv4 route table.
The BGP4 route table can contain a route with a next hop IPv4 address that is not reachable through an IGP route, even though the device can reach a hop farther away through an IGP route. This situation can occur when the IGPs do not learn a complete set of IGP routes. Instead, the device learns about an internal route through iBGP instead of through an IGP. In this case, the IPv4 route table does not contain a route that can be used to reach the BGP4 route destination.
When next-hop recursion is enabled, if the lookup for the next hop IP address finds an iBGP path that originated in the same autonomous system, then the next hop is considered resolved and BGP4-dependent routes are eligible for addition in the IPv4 route table.