ARPAnnouncement

• A purpose, not a packet shape: the packets are a specific use of the GratuitousARPRequest shape
• See GratuitousARPAmbiguousTerminology for how announcement and gratuitous ARP overlap
• Means: “I have this IP.” Sent after a successful ARPProbe as part of ARPDuplicateAddressDetection, after an IP change or failover, and by some devices as a keepalive. @ref
• Because it’s a request, a device that already owns that IP may reply. That’s how it can reveal an IP conflict during ARPDuplicateAddressDetection.