This NSE4 practice question tests your understanding of authentication and vpn. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
You are configuring a route-based IPsec VPN with BGP over the tunnel. After Phase 2 is up, the BGP session does not establish. You run 'diagnose debug ipsec' and see no errors. What should you check next?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Ensure the tunnel interface is added to the BGP neighbor configuration
In route-based VPN, the tunnel interface must be included in the BGP configuration as a neighbor update source or the BGP update must be allowed in the firewall policy.
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.
✗
Disable anti-replay on the tunnel
Why it's wrong here
Anti-replay would not affect BGP session establishment.
✗
Enable NAT traversal
Why it's wrong here
NAT traversal is for when one side is behind NAT, not for BGP.
✓
Ensure the tunnel interface is added to the BGP neighbor configuration
Why this is correct
BGP needs the interface to form the session; often the tunnel interface must be specified as the update source.
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 NSE4 OSPF questions on adjacency and route selection.
Authentication and VPN — This question tests Authentication and VPN — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Ensure the tunnel interface is added to the BGP neighbor configuration — In route-based VPN, the tunnel interface must be included in the BGP configuration as a neighbor update source or the BGP update must be allowed in the firewall policy.
What should I do if I get this NSE4 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 NSE4 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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Question Discussion
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