MCP IPv6 uses Neighbor Discovery in conjunction with the TCPIP NEIGHBOR command to discover routing information and manage neighbor nodes. Neighbor Discovery enables nodes to determine neighbors that are reachable and to find routers that are able to forward packets for them. Nodes can also use Neighbor Discovery to determine the data link-layer addresses for neighbors on attached links and to detect when these addresses change. The TCPIP NEIGHBOR command enables the administrator to add, modify, and delete a neighbor.
This section provides an overview of Neighbor Discovery and describes how to specify Neighbor Discovery options with the TCPIP OPTION command. For information on using the TCPIP NEIGHBOR command, refer to the Networking Commands and Inquiries Help.
Neighbor Discovery provides a means for resolving the following:
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Router Discovery
Router Discovery enables nodes to locate routers residing on a link and to determine the appropriate next hop. On multicast-capable links, each router periodically multicasts a router advertisement packet announcing its availability. Receipt of router advertisements from all routers facilitates the building of a list of default routers (routers to which packets can be sent) and address prefixes.
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Address Resolution
Address Resolution enables mapping from an IP address to a link-layer address. Neighbor address resolution, previously done through ARP in IPv4, is accomplished by multicasting a neighbor solicitation that asks the target node to return its link-layer address. When a node acknowledges that its link-layer address has changed, it multicasts a few unsolicited neighbor advertisement packets to all nodes to quickly update cached link-layer addresses that have become invalid.
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Neighbor Unreachability Detection
Neighbor Unreachability Detection determines that a neighbor is no longer reachable on a link. Communication to or through a neighbor can fail for numerous reasons at any time. If it is the path that has failed, because of a router failure, link or half-link failure, or because of a change in the link-layer address of a node, recovery might be possible. Therefore, a node actively tracks the reachability "state" for the neighbors to which it is sending packets.
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Redirection
Redirect messages are sent by routers to redirect a host to a better first-hop router for a specific destination or to inform hosts that a destination is in fact a neighbor (that is, on-link). Unlike IPv4, the recipient of an IPv6 redirect assumes that the new next-hop is on-link.
Neighbor Discovery is facilitated by the following five new ICMPv6 message types:
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Router Solicitation (Type 133)
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Router Advertisement (Type 134)
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Neighbor Solicitation (Type 135)
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Neighbor Advertisement (Type 136)
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Redirect (Type 137)
These messages are described in detail in RFC 2461, “Neighbor Discovery for IP Version 6 (IPv6).”
