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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)                 9  ICMP Message Types and Formats                      9  ICMP Version 6 (ICMPv6) Informational Message Types and Formats

ICMPv6 Neighbor Advertisement and Neighbor Solicitation Messages 1 2 ICMPv6 Router Renumbering Messages

Because of the different roles of routers and hosts in an IPv6 internetwork, hosts don't need to know very much about routes. They send datagrams intended for destinations on the local network directly, while those for other networks they dump to their local routers and let them “do the driving”, so to speak.

If a local network has only a single router, then it will send all such non-local traffic to that router. If it has more than one local router, the host then must decide which router to use for which traffic. In general terms, a host will not know the most efficient choice of router for every type of datagram it may need to send. In fact, many nodes start out with a limited routing table that says to send everything to a single default router, even if there are several routers on the network.

When a router receives datagrams destined for certain networks, it may realize that it would be more efficient if such traffic was sent by a host to a different router on the local network. If so, it will invoke the Redirect function by sending an ICMPv6 Redirect message to the device that sent the original datagram. This is the last of the functions that in IPv6 are performed by the Neighbor Discovery protocol, and is explained in a topic on the Redirect function in that section. Redirect messages are always sent unicast to the address of the device that originally sent the datagram that led to the Redirect being created.

ICMPv6 Neighbor Advertisement and Neighbor Solicitation Messages 1 2 ICMPv6 Router Renumbering Messages