The answer is that the inbound IPv6 ACL is blocking EIGRP query and reply packets, which prevents routes from being installed in the routing table. This occurs because EIGRP for IPv6 uses the multicast address FF02::A for all packet types, and while hello and update packets maintain the neighbor relationship, the Diffusing Update Algorithm (DUAL) requires query and reply packets to perform route computation and convergence. Without these packets, the router cannot complete the route installation process, even though the neighbor adjacency remains up. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how EIGRP ACL filtering can cause subtle routing failures—a common trap is assuming that a stable neighbor relationship guarantees route installation. Remember that EIGRP ACLs must permit all packet types (hello, update, query, reply) for full functionality. Memory tip: "Hello and Update keep the neighbor alive, but Query and Reply make the route arrive."
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A network engineer is troubleshooting an IPv6 connectivity issue on a router that is receiving routes via EIGRP for IPv6. The engineer notices that some routes are not being installed in the routing table, even though the EIGRP neighbor relationship is established. The engineer checks the interface configuration and finds an inbound IPv6 ACL that permits only certain EIGRP packets. The ACL permits EIGRP hello packets and updates, but not EIGRP queries or replies. What is the most likely cause of the missing routes?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL is blocking EIGRP query and reply packets, which are necessary for the EIGRP process to install routes.
EIGRP for IPv6 uses multicast address FF02::A for all EIGRP packets, including hello, update, query, and reply. The ACL permits only hello and update packets, blocking query and reply packets. EIGRP requires query and reply packets for the Diffusing Update Algorithm (DUAL) to converge and install routes; without them, the router cannot complete the route installation process, even though the neighbor relationship is established.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The ACL is blocking EIGRP query and reply packets, which are necessary for the EIGRP process to install routes.
Why this is correct
Correct because EIGRP queries and replies are used to ensure route consistency and convergence; blocking them can prevent route installation.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The EIGRP router ID is not configured.
Why it's wrong here
Incorrect because the neighbor relationship is established, indicating the router ID is valid.
✗
The ACL is applied outbound, blocking the EIGRP updates from being sent.
The EIGRP for IPv6 is not enabled on the interface.
Why it's wrong here
Incorrect because the neighbor relationship is established, meaning EIGRP is enabled.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that only hello and update packets are needed for EIGRP route installation, but the trap here is that query and reply packets are essential for DUAL convergence and route installation, even when neighbors are up.
Detailed technical explanation
How to think about this question
EIGRP for IPv6 uses protocol number 88 and multicast address FF02::A for all packet types. The ACL filters based on the IPv6 next header field, which is 88 for EIGRP; however, it cannot differentiate between packet types (hello, update, query, reply) at Layer 3, so the ACL must permit all EIGRP traffic or use a more granular method like NBAR. In practice, blocking query and reply packets can cause stuck-in-active routes and prevent DUAL from achieving a feasible successor, leading to incomplete routing tables.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL is blocking EIGRP query and reply packets, which are necessary for the EIGRP process to install routes. — EIGRP for IPv6 uses multicast address FF02::A for all EIGRP packets, including hello, update, query, and reply. The ACL permits only hello and update packets, blocking query and reply packets. EIGRP requires query and reply packets for the Diffusing Update Algorithm (DUAL) to converge and install routes; without them, the router cannot complete the route installation process, even though the neighbor relationship is established.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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