10.2 IPv6 Neighbor Discovery

A major difference between IPv4 and IPv6 involves how IPv6 hosts learn their own addresses and learn about their neighbors, including other hosts and routers. Neighbor Discovery Protocol, also known as ND or NDP, facilitates this and other key functions. ND is defined in RFC 2461. The remainder of this section introduces ND functionality, lists its main features, and then lists the related ICMPv6 messages, which are beyond the scope of the exam but are useful for study and reference.

In IPv6 networks, ND Protocol uses ICMPv6 messages and solicited-node multicast addresses for its core functions, which center on discovering and tracking other IPv6 hosts on connected interfaces. ND is also used for address autoconfiguration.

Major roles of IPv6 ND include the following:

  • Stateless address autoconfiguration (detailed in RFC 2462)

  • Duplicate address detection (DAD)

  • Router discovery

  • Prefix discovery

  • Parameter discovery (link MTU, hop limits)

  • Neighbor discovery

  • Neighbor address resolution (replaces ARP, both dynamic and static)

  • Neighbor and router reachability verification

ND uses five types of ICMPv6 messages to do its work as shown below:

ND Functions in IPv6

Message Type Information Sought or Sent Source Address Destination Address ICMP Type, Code
Router Advertisement (RA) Routers advertise their presence and link prefixes, MTU, and hop limits. Router's link-local address FF02::1 for periodic broadcasts; address of querying host for responses to an RS 134, 0
Router Solicitation (RS) Hosts query for the presence of routers on the link. Address assigned to querying interface, if assigned, or :: if not assigned FF02::2 133, 0
Neighbor Solicitation (NS) Hosts query for other nodes' link-layer addresses. Used for duplicate address detection and to verify neighbor reachability. Address assigned to querying interface, if assigned, or :: if not assigned Solicited-node multicast address or the target node's address, if known 135, 0
Neighbor Advertisement (NA) Sent in response to NS messages and periodically to provide information to neighbors. Configured or automatically assigned address of originating interface Address of node requesting the NA or FF02::1 for periodic advertisements 136, 0
Redirect Sent by routers to inform nodes of better next-hop routers. Link-local address of originating node Source address of requesting node 137, 0

Neighbor Advertisements

IPv6 nodes send Neighbor Advertisement (NA) messages periodically to inform other hosts on the same network of their presence and link-layer addresses.

Neighbor Solicitation

IPv6 nodes send NS messages to find the link-layer address of a specific neighbor. This message is used in three operations:

  • Duplicate address detection

  • Neighbor reachability verification

  • Layer 3 to Layer 2 address resolution (as a replacement for ARP)

IPv6 does not include ARP as a protocol but rather integrates the same functionality into ICMP as part of neighbor discovery. The response to an NS message is an NA message.