Neighbor Discovery (ND)

Neighbor Discovery (ND) is the IPv6 equivalent of the IPv4 ARP for layer 2 address resolution. ND uses IPv6 ICMP messages to provide for discovery of IPv6 devices such as other switches, routers, management stations, and servers on the same interface. ND runs automatically in the default configuration and provides services along with those provided in IPv4 by ARP. For example:

  • Determine the link-layer address of neighbors on the same VLAN interface.

  • Verify that a neighbor is reachable.

  • Track neighbor (local) routers.

Neighbor Discovery enables functions such as the following:

  • router and neighbor solicitation and discovery

  • detecting address changes for devices on a VLAN

  • identifying a replacement for a router or router path that has become unavailable

  • duplicate address detection (DAD)

  • RA processing

  • neighbor reachability

  • autoconfiguration of unicast addresses

  • resolution of destination addresses

  • changes to link-layer addresses

An instance of ND is triggered on a device when a new (tentative) or changed IPv6 address is detected. (Includes stateless, stateful, and static address configuration.) ND operates in a per-VLAN scope, that is, within the VLAN on which the device running the ND instance is a member. ND actually occurs when there is communication between devices on a VLAN. That is, a device needing to determine the link-layer address of another device on the VLAN initiates a (multicast) neighbor solicitation message (containing a solicited-node multicast address that corresponds to the IPv6 address of the destination device) on the VLAN. When the destination device receives the neighbor solicitation, it responds with a neighbor advertisement message identifying its link-layer address. When the initiating device receives this advertisement, the two devices are ready to exchange traffic on the VLAN interface. Also, when an IPv6 interface becomes operational, it transmits a router solicitation on the interface and listens for an RA.


Neighbor and router solicitations must originate on the same VLAN as the receiving device. To support this operation, IPv6 is designed to discard any incoming neighbor or router solicitation that does not have a value of 255 in the IP Hop Limit field. For a complete list of requirements, see RFC 2460.

When a pair of IPv6 devices in a VLAN exchange communication, they enter each other's IPv6 and corresponding MAC addresses in their respective neighbor caches. These entries are maintained for a time after communication ceases and then dropped.

To view or clear the content of the neighbor cache, see Viewing the Neighbor Cache.

For related information, see RFC 2461: "Neighbor Discovery for IP Version 6 (IPv6)."