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.
R1 and R2 have an IPsec VPN tunnel between their physical interfaces. They are running BGP over the tunnel interface. R1's show ip bgp summary shows the BGP session with R2 as established, but R1's show ip bgp shows no routes from R2. R2's show ip bgp shows routes from R1. 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
✓
R1 has a route-map applied to the BGP neighbor inbound that denies all routes.
The correct answer is A because R1's BGP session is established (TCP port 179 is up), but no routes are received from R2. This indicates that an inbound route-map on R1 is filtering all incoming BGP updates. The route-map is applied to the neighbor inbound direction, which matches the symptom: R1 sees the session as established but has zero routes from R2, while R2 sees routes from R1 (since outbound filtering on R2 is not the issue).
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.
✓
R1 has a route-map applied to the BGP neighbor inbound that denies all routes.
Why this is correct
An inbound route-map can filter all incoming BGP updates, preventing routes from being installed in the BGP table.
Related concept
Read the scenario before looking for a memorised answer.
✗
BGP next-hop-self is missing on R2.
Why it's wrong here
Missing next-hop-self would cause R1 to not have a route to the next-hop, but the routes would still appear in BGP table as not best.
✗
The IPsec tunnel is not encrypting BGP traffic.
Why it's wrong here
BGP session is established, so encryption is working.
✗
R2 is not advertising any networks.
Why it's wrong here
R2 sees routes from R1, so it is advertising.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between a BGP session being established (TCP state) and routes being exchanged (NLRI processing), so candidates may incorrectly assume that an established session guarantees route exchange, overlooking inbound route-map filtering.
Detailed technical explanation
How to think about this question
BGP route-maps applied inbound on a neighbor can filter prefixes using match statements (e.g., match ip address prefix-list). Even if the BGP session is established, a deny statement in the route-map will cause the router to discard received updates before they enter the BGP table. This is a common troubleshooting scenario: 'show ip bgp neighbors X.X.X.X received-routes' would reveal the filtered prefixes, while 'show ip bgp' shows none. In real-world deployments, such filtering is used for policy-based routing or to enforce prefix limits.
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: R1 has a route-map applied to the BGP neighbor inbound that denies all routes. — The correct answer is A because R1's BGP session is established (TCP port 179 is up), but no routes are received from R2. This indicates that an inbound route-map on R1 is filtering all incoming BGP updates. The route-map is applied to the neighbor inbound direction, which matches the symptom: R1 sees the session as established but has zero routes from R2, while R2 sees routes from R1 (since outbound filtering on R2 is not the issue).
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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