Quick Answer
The answer is the EIGRP Acknowledgment (Ack) packet. This is correct because the Ack packet is a special, data-less packet used exclusively by EIGRP’s Reliable Transport Protocol (RTP) to confirm the receipt of reliable packets like Update, Query, and Reply. Unlike other EIGRP packet types, an Ack is always sent as a unicast back to the source router and contains no routing information—its sole purpose is to serve as a delivery confirmation, ensuring ordered and guaranteed delivery of critical topology changes. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of EIGRP reliability mechanisms and often appears in questions about packet types or RTP operation. A common trap is confusing the Ack with the Hello packet, since both are lightweight, but remember that Hellos are multicast for neighbor discovery while Acks are unicast and triggered only by reliable packets. A helpful memory tip: “Ack is a unicast ‘thanks’ with no data—just a receipt.”
This 300-410 practice question tests your understanding of device access control. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which EIGRP packet type is used to acknowledge receipt of a reliable packet?
Hello
Why wrong: Hello packets are used for neighbor discovery and keepalive, not for acknowledgment.
Update
Why wrong: Update packets carry routing information and are sent reliably, but they are not used for acknowledgment.
Reply
Why wrong: Reply packets are sent in response to Query packets, not as general acknowledgments.
Ack
Correct. The Ack packet is an empty packet used to confirm receipt of reliable packets.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Ack
D is correct because the EIGRP Ack (Acknowledgement) packet is a special packet used exclusively to confirm the reliable delivery of EIGRP packets such as Update, Query, and Reply. Ack packets are sent as unicast to the source router and contain no data, serving only as a delivery confirmation. This mechanism ensures that EIGRP's Reliable Transport Protocol (RTP) can guarantee ordered and guaranteed delivery of critical routing information.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Hello
Why it's wrong here
Hello packets are used for neighbor discovery and keepalive, not for acknowledgment.
Update
Why it's wrong here
Update packets carry routing information and are sent reliably, but they are not used for acknowledgment.
Reply
Why it's wrong here
Reply packets are sent in response to Query packets, not as general acknowledgments.
Ack
Why this is correct
Correct. The Ack packet is an empty packet used to confirm receipt of reliable packets.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Cisco often tests the distinction between packet types that are sent reliably versus unreliably, and the trap here is that candidates confuse the Reply packet (which is a response to a Query) with an acknowledgment, when in fact Reply packets are data-carrying reliable packets that themselves require an Ack.
Detailed technical explanation
EIGRP's Reliable Transport Protocol (RTP) uses sequence numbers to track reliable packets; when a router receives a reliable packet (e.g., Update, Query, Reply), it must respond with an Ack packet containing the same sequence number to confirm receipt. If the sender does not receive an Ack within the retransmission timeout (RTO), it will retransmit the packet, up to a maximum of 16 times (by default). In real-world scenarios, missing Acks can indicate network congestion or asymmetric routing issues, leading to neighbor flapping or route instability.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
The correct answer is: Ack — D is correct because the EIGRP Ack (Acknowledgement) packet is a special packet used exclusively to confirm the reliable delivery of EIGRP packets such as Update, Query, and Reply. Ack packets are sent as unicast to the source router and contain no data, serving only as a delivery confirmation. This mechanism ensures that EIGRP's Reliable Transport Protocol (RTP) can guarantee ordered and guaranteed delivery of critical routing information.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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