Quick Answer

The answer is ACK, which acknowledges receipt of a reliable packet. This is correct because EIGRP uses five distinct packet types to manage neighbor relationships and route exchange, with ACK packets serving as the lightweight, unreliable confirmation that a reliable packet—such as an Update, Query, or Reply—was received successfully. On the ENCOR 350-401 exam, you will often see a drag-and-drop task matching each packet type to its function, testing your ability to distinguish between Hello (neighbor discovery), Update (routing information), Query (alternate path request), Reply (response to Query), and ACK (reliable delivery acknowledgment). A common trap is confusing ACK with Hello, but remember that Hello packets are used for neighbor discovery and maintenance, while ACKs are purely confirmations and contain no routing data. A useful memory tip: “ACK is the receipt, Hello is the greeting, Update is the news, Query asks, Reply answers.”