The answer is that the IPsec tunnel is not encrypting EIGRP multicast traffic, causing queries to be dropped. This is correct because EIGRP relies on the multicast address 224.0.0.10 for sending queries and replies during the Diffusing Update Algorithm (DUAL) process; if the IPsec crypto ACL only matches unicast or specific protocols, the multicast EIGRP packets are not encrypted and are subsequently dropped by the remote router’s IPsec process, leaving queries unanswered. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IPsec tunnel misconfigurations can break multicast-dependent routing protocols, often appearing as a trick where the neighbor stays up (due to unicast keepalives) but routes become stuck-in-active. A common trap is assuming the tunnel is fully functional because the neighbor state is established, but the real issue lies in the crypto ACL’s failure to include EIGRP’s multicast group. Memory tip: “Multicast must match the crypto map—if EIGRP’s 224.0.0.10 is missing, SIA is your prize.”
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
R1 and R2 are connected via an IPsec VPN tunnel. They are running EIGRP over the tunnel. R1's show ip eigrp neighbors shows R2 as up, but R1's show ip eigrp topology shows a route from R2 as 'stuck-in-active' (SIA). R1's show ip eigrp traffic shows queries being sent but no replies. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The IPsec tunnel is not encrypting EIGRP multicast traffic, causing queries to be dropped.
The correct answer is A because EIGRP uses multicast address 224.0.0.10 for neighbor discovery and route updates. If the IPsecVPN tunnel is misconfigured to not encrypt multicast traffic (e.g., using an ACL that only matches unicast or specific protocols), the EIGRP queries sent as multicast packets will be dropped by the IPsec process or the remote router, preventing replies. This results in the neighbor appearing up (due to unicast keepalives or a stale state) but routes stuck-in-active because queries are never answered.
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 IPsec tunnel is not encrypting EIGRP multicast traffic, causing queries to be dropped.
Why this is correct
EIGRP queries are sent to multicast address 224.0.0.10. If the crypto map does not protect multicast traffic, queries may be dropped, leading to SIA.
Related concept
Read the scenario before looking for a memorised answer.
✗
EIGRP split-horizon is enabled on the tunnel interface.
Why it's wrong here
Split-horizon affects route advertisement, not query/reply process.
✗
The EIGRP active timer is set too low.
Why it's wrong here
A low active timer would cause SIA faster, but the root cause is missing replies.
✗
R2 has a passive interface for the tunnel.
Why it's wrong here
Passive interface would prevent R2 from sending hellos, but neighbor is up.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a neighbor being 'up' in 'show ip eigrp neighbors' guarantees full EIGRP communication, when in fact the neighbor state can remain up due to unicast keepalives or a stale hold timer while multicast-based queries are blocked by IPsec ACLs.
Detailed technical explanation
How to think about this question
EIGRP relies on reliable multicast for queries during the active process; if IPsec is configured with a crypto ACL that only matches unicast traffic (e.g., using 'permit ip host X host Y' instead of 'permit ip any any' or a multicast-specific entry), multicast packets like EIGRP hellos and queries are sent in the clear or dropped. In real-world deployments, this often occurs when the crypto ACL is too restrictive, causing partial connectivity where unicast keepalives (if configured) or stale neighbor states persist, but multicast-based EIGRP operations fail.
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.
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The IPsec tunnel is not encrypting EIGRP multicast traffic, causing queries to be dropped. — The correct answer is A because EIGRP uses multicast address 224.0.0.10 for neighbor discovery and route updates. If the IPsec VPN tunnel is misconfigured to not encrypt multicast traffic (e.g., using an ACL that only matches unicast or specific protocols), the EIGRP queries sent as multicast packets will be dropped by the IPsec process or the remote router, preventing replies. This results in the neighbor appearing up (due to unicast keepalives or a stale state) but routes stuck-in-active because queries are never answered.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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