Router Advertisement RA, IPv6
IPv6 packet the router sends (multicast, every ~7-200 seconds) announcing its presence + prefix + flags (DHCPv6 needed?).
Contains: Source link-layer address (MAC), prefix info (subnet for SLAAC), DNS servers (RDNSS option, RFC 8106), MTU, hop limit. Clients can trigger an RA by sending a Router Solicitation (RS). RA Guard (Cisco feature) only allows RA from specific ports — defence against rogue routers.