- A
Hello packets are sent unreliably and do not require an acknowledgment.
Correct because Hello packets are sent as best-effort (unreliable) and are not acknowledged.
- B
Update packets are always sent as multicast to all EIGRP neighbors.
Why wrong: Incorrect because Update packets can be unicast when responding to a Query or when sending a partial update to a specific neighbor.
- C
Query packets are sent reliably and require a Reply from each neighbor.
Correct because Query packets are reliable (require acknowledgment) and each neighbor must respond with a Reply.
- D
ACK packets are unicast and are used to acknowledge reliable EIGRP packets.
Correct because ACK packets are sent as unicast to acknowledge receipt of reliable packets like Update, Query, and Reply.
- E
Reply packets are sent unreliably to conserve bandwidth.
Why wrong: Incorrect because Reply packets are sent reliably (require acknowledgment) to ensure the querying router receives the information.
Quick Answer
The correct answer is that ACK packets are unicast and used to acknowledge reliable EIGRP packets. This is true because EIGRP uses five distinct packet types—Hello, Update, Query, Reply, and ACK—each with a specific role in the protocol’s operation. Hello packets are multicast to 224.0.0.10 for neighbor discovery and are unacknowledged, while Update and Query packets are sent reliably and require an ACK in return. On the ENCOR 350-401 exam, this topic tests your understanding of EIGRP’s reliability mechanism and the difference between multicast and unicast delivery. A common trap is confusing Hello packets with Update packets, since both can be multicast, but only Update packets trigger an ACK. Remember the mnemonic “HUQRA” (Hello, Update, Query, Reply, ACK) to keep the five types straight, and note that ACK is the only one that is always unicast.
CCNP EIGRP Practice Question
This 350-401 practice question tests your understanding of eigrp. 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 three statements about EIGRP packet types are true? (Choose three.)
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Hello packets are sent unreliably and do not require an acknowledgment.
EIGRP uses five packet types: Hello (neighbor discovery/maintenance), Update (route information), Query (ask neighbors for routes), Reply (response to Query), and ACK (acknowledgment). Hello packets are multicast to 224.0.0.10, unacknowledged, and used to form and maintain adjacencies. Update packets are sent reliably (with ACK) and can be multicast or unicast. Query packets are multicast and require a Reply. ACK packets are unicast and are used to acknowledge reliable packets.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Hello packets are sent unreliably and do not require an acknowledgment.
Why this is correct
Correct because Hello packets are sent as best-effort (unreliable) and are not acknowledged.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
Update packets are always sent as multicast to all EIGRP neighbors.
Why it's wrong here
Incorrect because Update packets can be unicast when responding to a Query or when sending a partial update to a specific neighbor.
- ✓
Query packets are sent reliably and require a Reply from each neighbor.
Why this is correct
Correct because Query packets are reliable (require acknowledgment) and each neighbor must respond with a Reply.
Related concept
OSPF neighbours must agree on key parameters.
- ✓
ACK packets are unicast and are used to acknowledge reliable EIGRP packets.
Why this is correct
Correct because ACK packets are sent as unicast to acknowledge receipt of reliable packets like Update, Query, and Reply.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
Reply packets are sent unreliably to conserve bandwidth.
Why it's wrong here
Incorrect because Reply packets are sent reliably (require acknowledgment) to ensure the querying router receives the information.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 350-401 question test?
EIGRP — This question tests EIGRP — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Hello packets are sent unreliably and do not require an acknowledgment. — EIGRP uses five packet types: Hello (neighbor discovery/maintenance), Update (route information), Query (ask neighbors for routes), Reply (response to Query), and ACK (acknowledgment). Hello packets are multicast to 224.0.0.10, unacknowledged, and used to form and maintain adjacencies. Update packets are sent reliably (with ACK) and can be multicast or unicast. Query packets are multicast and require a Reply. ACK packets are unicast and are used to acknowledge reliable packets.
What should I do if I get this 350-401 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.
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