Neighbor Discovery (ND)
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4/8/2010
IPv6 Neighbor Discovery (ND) is a series of five Internet Control Message Protocols for IPv6 (ICMPv6) messages that manage node-to-node communication on a link. Nodes on the same link are also called neighboring nodes.
ND replaces Address Resolution Protocol (ARP), ICMP Router Discovery, and ICMP Redirect, which are used in IPv4. It also provides additional functionality. ND is described in RFC 2461.
The following list shows how hosts, routers, and nodes use ND:
- Hosts use ND to discover neighboring routers, addresses, address prefixes, and other parameters.
- Routers use ND to advertise their presence, host parameters, and on-link prefixes. Routers also use ND to inform hosts of a better next-hop address to forward packets for a specific destination.
- Nodes use ND to resolve the link-layer address of a neighboring node to which an IPv6 packet is being forwarded. Nodes also use ND to determine when the link-layer address of a neighboring node has changed, and whether IPv6 packets can be sent to and received from a neighbor.
The following table shows ND processes.
| Process | Description |
|---|---|
Router discovery |
The process by which a host discovers the local routers on an attached link and automatically configures a default router. In IPv4, this is equivalent to using ICMPv4 router discovery to configure a default gateway. |
Prefix discovery |
The process by which a host discovers the network prefixes for local destinations. |
Parameter discovery |
The process by which a host discovers additional operating parameters, including the link maximum transmission unit (MTU) and the default hop limit for outbound packets. |
Address autoconfiguration |
The process for configuring IP addresses for interfaces in the absence of a server that configures stateful addresses, such as Dynamic Host Configuration Protocol version 6 (DHCPv6). For more information, see IPv6 Address Autoconfiguration. |
Address resolution |
The process by which a node resolves a neighboring node's IPv6 address to its link-layer address. The resolved link-layer address becomes an entry in a neighbor cache in the node. The link layer address is equivalent to ARP in IPv4, and the neighbor cache is equivalent to the ARP cache. The neighbor cache displays the interface identifier for the neighbor cache entry, the neighboring node IPv6 address, the corresponding link-layer address, and the state of the neighbor cache entry. |
Next-hop determination |
The process by which a node determines the IPv6 address of the neighbor to which a packet is being forwarded. The determination is made based on the destination address. The forwarding or next-hop address is either the destination address of the packet being sent or the address of a neighboring router. The resolved next-hop address for a destination becomes an entry in a node's destination cache, also known as a route cache. The route cache displays the destination address, the interface identifier and next-hop address, the interface identifier and address used as a source address when sending to the destination, and the path MTU for the destination. |
Neighbor unreachability detection |
The process by which a node determines that neighboring hosts or routers are no longer available on the local network segment. After the link-layer address for a neighbor has been determined, the state of the entry in the neighbor cache is tracked. If the neighbor is no longer receiving and sending back packets, the neighbor cache entry is eventually removed. |
Duplicate address detection |
The process by which a node determines that an address considered for use is not already in use by a neighboring node. This is equivalent to the use of gratuitous ARP frames in IPv4. |
Redirect function |
The process by which a router informs a host of a better first-hop IPv6 address to reach a destination. This is equivalent to the function of the IPv4 ICMP Redirect message. |