The NEXT_HOP attribute indicates the IPv4 address of the BGP router that is the next-hop to reach the IPv4 prefixes in the NLRI field. If the Update message is advertising routes other than IPv4 unicast routes the next-hop of these routes is encoded in the MP_REACH_NLRI attribute. For more details, see Multi-protocol BGP attributes.
The rules for deciding what next-hop address types to accept in a received BGP route and what next-hop address types to advertise as a BGP next-hop are address family dependent. The following sections summarize the key details.