Neighbor Solicitation and Neighbor Advertisement
Neighbor Discovery uses ICMPv6 instead of ARP. For initial address resolution, a host sends a Neighbor Solicitation (NS) to the target address’s solicited-node multicast group. The target returns a Neighbor Advertisement (NA) with its link-layer address. The requester uses the result to fill its neighbor cache.
NS and NA messages also help check whether known neighbors are reachable. Duplicate Address Detection uses a solicitation with an unspecified source to check a tentative address before assigning it.
Router Solicitation and Router Advertisement
A host sends a Router Solicitation (RS) to ask local routers for information without waiting for the next periodic announcement. Routers send Router Advertisements (RAs) periodically and in response to solicitations. An RA can identify a default router through its router lifetime and supply prefix and link parameters.
A host can use a prefix marked for autonomous configuration to form an address with SLAAC. The prefix information and default-router lifetime serve different purposes. A router can advertise a usable prefix without offering itself as a default router.
Try it in a lab
- On an IPv6 LAN with a router, capture a host joining the link. Find the Router Solicitation and Advertisement and compare the router lifetime with the prefix flags.
- Ping a local IPv6 peer whose MAC address is not cached. Find the Neighbor Solicitation and Advertisement, then compare the advertised link-layer address with the host’s neighbor cache.