With recursive next-hop lookups, a device can find the IGP route to the next-hop gateway for a BGP4+ route.
For each BGP4+ route learned, the device performs a route lookup to obtain the IPv6 address of the next hop for the route. A BGP4+ route is eligible for addition in the IPv6 route table only if the following conditions are true:
By default, the software performs only one lookup for the next-hop IPv6 address for the BGP4+ route. If the next-hop lookup does not result in a valid next-hop IPv6 address, or if the path to the next-hop IPv6 address is a BGP4+ path, the BGP4+ route destination is considered unreachable. The route is not eligible to be added to the IPv6 route table.
The BGP4+ route table can contain a route with a next-hop IPv6 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, so the device learns about an internal route through IBGP instead of through an IGP. In this case, the IPv6 route table does not contain a route that can be used to reach the BGP4+ route destination.
You can use recursive next-hop lookups to enable the device to find the IGP route to the next-hop gateway for a BGP4+ route. With this feature enabled, if the first lookup for a BGP4+ route results in an IBGP path that originated in the same AS, rather than in an IGP path or static route path, the device performs a lookup on the next-hop IPv6 address for the next-hop gateway.
If the second lookup results in an IGP path, the software considers the BGP4+ route to be valid and adds it to the IPv6 route table. Otherwise, the device performs another lookup on the next-hop IPv6 address of the next hop for the next-hop gateway, and so on, until one of the lookups results in an IGP route.
Note
You must configure a static route or use an IGP to learn the route to the EBGP multihop peer.