Question 1,810 of 2,152

IPsec Site-to-Site VPN →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the crypto map access list does not match GRE protocol (47) traffic. This is the most likely cause because when a GRE tunnel runs over IPsec, the crypto map applied to the physical interface must explicitly permit GRE packets (protocol 47) in its access list; otherwise, the GRE traffic is sent unencrypted and the remote router drops it, breaking the routing protocol adjacency even though the GRE tunnel itself shows up/up. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IPsec interacts with overlay tunnels—a common trap is assuming a working GRE tunnel guarantees encrypted transport, when in fact the crypto map’s ACL is the gatekeeper. Remember the memory tip: “GRE is 47, so match it in the ACL to keep the adjacency from heaven.”

300-410 IPsec Site-to-Site VPN Practice Question

This 300-410 practice question tests your understanding of ipsec site-to-site vpn. 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 IPsec site-to-site VPN that uses a GRE tunnel over IPsec. The GRE tunnel is up/up, but the routing protocol (EIGRP) running over the GRE tunnel is not forming an adjacency. The engineer checks the tunnel configuration and sees that the tunnel source and destination are correct. What is the most likely cause?

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.

Question 1mediummultiple choice

Study the full EIGRP explanation →

A
The crypto map access list does not match GRE protocol (47) traffic.
Correct because GRE uses protocol 47; if the crypto map's access list only matches IP traffic between the LAN subnets, the GRE packets themselves are not encrypted and are dropped, causing the GRE tunnel to appear up but the routing protocol to fail.
B
The EIGRP hello timer is set too high.
Why wrong: Incorrect because a high hello timer would cause slow convergence, not a complete failure to form adjacency.
C
The tunnel interface is not configured with an IP address.
Why wrong: Incorrect because the tunnel is up/up, so it must have an IP address.
D
The IPsec transform set does not include ESP encryption.
Why wrong: Incorrect because the transform set is used for IPsec, but the issue is that GRE packets are not being encrypted at all.

Full breakdown with real-world context →

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

✓

The crypto map access list does not match GRE protocol (47) traffic.

For a GRE over IPsec tunnel, the GRE tunnel must be protected by the crypto map. If the crypto map is applied to the physical interface but the GRE tunnel traffic is not matched by the crypto map's access list, the GRE packets will be sent unencrypted and the remote router will drop them, preventing the routing protocol from forming an adjacency.

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.

✓
The crypto map access list does not match GRE protocol (47) traffic.
Why this is correct
Correct because GRE uses protocol 47; if the crypto map's access list only matches IP traffic between the LAN subnets, the GRE packets themselves are not encrypted and are dropped, causing the GRE tunnel to appear up but the routing protocol to fail.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The EIGRP hello timer is set too high.
Why it's wrong here
Incorrect because a high hello timer would cause slow convergence, not a complete failure to form adjacency.
✗
The tunnel interface is not configured with an IP address.
Why it's wrong here
Incorrect because the tunnel is up/up, so it must have an IP address.
✗
The IPsec transform set does not include ESP encryption.
Why it's wrong here
Incorrect because the transform set is used for IPsec, but the issue is that GRE packets are not being encrypted at all.

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 300-410 OSPF questions on adjacency and route selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The crypto map access list does not match GRE protocol (47) traffic. — For a GRE over IPsec tunnel, the GRE tunnel must be protected by the crypto map. If the crypto map is applied to the physical interface but the GRE tunnel traffic is not matched by the crypto map's access list, the GRE packets will be sent unencrypted and the remote router will drop them, preventing the routing protocol from forming an adjacency.

What should I do if I get this 300-410 question wrong?

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?

OSPF neighbours must agree on key parameters.

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