The NEXT_HOP path attribute defines the IP address of the border router that should be used as the next hop to the destinations listed in the UPDATE message. If a border router belongs to the same AS as its peer, then the peer is an internal border router. Otherwise, it is an external border router. A BGP speaker can advertise any internal border router as the next hop provided that the interface associated with the IP address of this border router (as specified in the NEXT_HOP path attribute) shares a common subnet with both the local and remote BGP speakers. A BGP speaker can advertise any external border router as the next hop, provided that the IP address of this border router was learned from one of the BGP speaker's peers, and the interface associated with the IP address of this border router (as specified in the NEXT_HOP path attribute) shares a common subnet with the local and remote BGP speakers. A BGP speaker needs to be able to support disabling advertisement of external border routers.
A BGP speaker must never advertise an address of a peer to that peer as a NEXT_HOP, for a route that the speaker is originating. A BGP speaker must never install a route with itself as the next hop.
When a BGP speaker advertises the route to a BGP speaker located in its own autonomous system, the advertising speaker shall not modify the NEXT_HOP attribute associated with the route. When a BGP speaker receives the route via an internal link, it may forward packets to the NEXT_HOP address if the address contained in the attribute is on a common subnet with the local and remote BGP speakers.