Which type of message is used to request the link-layer address of a neighbor?

The type of message used to request the link-layer address of a neighbor is the Neighbor Solicitation message. In the context of IPv6 networking, Neighbor Solicitation messages are part of the Neighbor Discovery Protocol (NDP). This protocol helps devices on the same local link discover each other's presence, as well as resolve link-layer addresses corresponding to known IPv6 addresses.

When a device needs to determine the link-layer address (such as a MAC address) of another device identified by its IPv6 address, it sends out a Neighbor Solicitation message. This message is typically broadcast on the local link to solicit a response from the specific IPv6 address that the sender wishes to resolve. The device with the matching IPv6 address responds with a Neighbor Advertisement message, which includes its link-layer address.

The other options represent different types of messages within the Neighbor Discovery Protocol or related functions but serve different purposes. Router Advertisements, for example, are sent by routers to announce their presence and provide routing information to hosts on the local network. Router Solicitation messages are sent by hosts to request Router Advertisements. Understanding these distinctions helps clarify the role of Neighbor Solicitation in the context of link-layer address resolution.