IPv6 Network - Global and Link-Local Next Hop Addresses

Although this isn’t apparent when looking at the BGP routing table, multiprotocol BGP actually employs two next hop addresses: a global scope address, which functions just as the IPv4 next hop address, and a link-local next hop address. The reason for this has to do with ICMP redirects, which are used to avoid unnecessary router hops. When a router receives a packet on an interface, and this packet is forwarded out of the same interface, either to its destination or to another router, the source could just as easily have sent this packet directly to the destination or router in question, as it’s on the same subnet. The redirect message is used to tell the source about this so that subsequent packets skip the unnecessary hop. Because the IPv6 standards require ICMP messages to point to the link-local address of the next hop router, all routing protocols must exchange link-local addresses. (When the redirect points to the ultimate destination of the packet, it contains the destination address and not a link-local address.).

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