Which ICMPv6 message is sent by a host to determine first-hop devices upon joining a network?

• A. Router Advertisement
• B. Router Solicitation
• C. Neighbor Solicitation
• D. Echo Request

Answer: (B)

When an IPv6 host joins a network, it needs to learn who the first-hop routers are. It does this using the Router Solicitation message, sent to the all-routers multicast address to prompt any on-link routers to identify themselves. Routers respond with Router Advertisement messages that announce their presence and provide network parameters like the default route and prefix information. The other message types serve different purposes: Router Advertisement is the router’s unsolicited advertisement, Neighbor Solicitation is for discovering or verifying link-layer addresses of neighbors, and Echo Request is used for ping. So the host’s Router Solicitation is the mechanism that initiates discovery of the first-hop devices.