Layer 2 mapping information is announced by an interface without receiving a request for the mapping first. Which term describes this event?

ICMP operates at Layer 3 and is used for network diagnostics and error reporting, not for mapping or announcing Layer 2 MAC address information.

STP is a Layer 2 loop-prevention protocol that manages port states on switches; it has no role in IP-to-MAC address mapping or announcement.

Gratuitous ARP is sent by a host using its own IP address as both the sender and target in an ARP reply, broadcast onto the local segment without any prior request. It is used to proactively update neighbor ARP caches, detect duplicate IP addresses, or announce a new MAC address after a NIC replacement or failover event.

Standard ARP is a request-response exchange where a host first broadcasts a request asking which MAC address owns a given IP before any reply is generated; it is not unsolicited.