- A
The BGP synchronization rule is enabled, and the IGP does not carry the BGP routes, preventing installation.
Why wrong: Synchronization is disabled by default in modern IOS; even if enabled, it would affect iBGP routes only if the IGP does not have the prefix, but the issue is next-hop reachability.
- B
The next-hop address for the BGP routes is the physical interface IP of the remote router, which is not reachable through the tunnel, so the route is not installed.
iBGP does not change the next hop by default. If the BGP session is over the tunnel, but the next hop is the physical IP, the router cannot reach it unless the IGP or a static route points to the tunnel. The fix is to use next-hop-self on the neighbor.
- C
The IPsec transform set uses SHA-2 authentication, which is incompatible with BGP MD5 authentication.
Why wrong: BGP MD5 authentication is separate from IPsec and does not interfere with route installation.
- D
The BGP session is using loopback interfaces, and the IPsec tunnel is not configured to encrypt traffic to the loopback.
Why wrong: If the BGP session is over loopbacks, the IPsec tunnel must encrypt traffic between the loopbacks; but the issue is next-hop reachability, not encryption.
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site 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.
An engineer configures an IPsec site-to-site VPN between two routers using iBGP for routing. The BGP session comes up, but routes learned from the remote site are not installed in the routing table. The engineer verifies that the IPsec tunnel is up and that the BGP prefixes are present in the BGP table. What is the most likely explanation?
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 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 next-hop address for the BGP routes is the physical interface IP of the remote router, which is not reachable through the tunnel, so the route is not installed.
In iBGP, the next-hop for routes learned from an eBGP peer is not changed by default. When the remote router advertises prefixes, it sets the next-hop to its physical interface IP address. If the IPsec tunnel is configured to encrypt traffic between the two routers' tunnel endpoints (e.g., virtual tunnel interfaces or crypto maps applied to physical interfaces), the physical interface IP of the remote router may not be reachable through the tunnel. BGP will not install a route in the routing table if the next-hop is not reachable via a valid route in the routing table, even if the BGP session is up and the prefixes are in the BGP table.
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 BGP synchronization rule is enabled, and the IGP does not carry the BGP routes, preventing installation.
Why it's wrong here
Synchronization is disabled by default in modern IOS; even if enabled, it would affect iBGP routes only if the IGP does not have the prefix, but the issue is next-hop reachability.
- ✓
The next-hop address for the BGP routes is the physical interface IP of the remote router, which is not reachable through the tunnel, so the route is not installed.
Why this is correct
iBGP does not change the next hop by default. If the BGP session is over the tunnel, but the next hop is the physical IP, the router cannot reach it unless the IGP or a static route points to the tunnel. The fix is to use next-hop-self on the neighbor.
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 IPsec transform set uses SHA-2 authentication, which is incompatible with BGP MD5 authentication.
- ✗
The BGP session is using loopback interfaces, and the IPsec tunnel is not configured to encrypt traffic to the loopback.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept that BGP route installation depends on next-hop reachability, and candidates mistakenly assume that a working BGP session and IPsec tunnel guarantee route installation, ignoring the need for the next-hop to be reachable via the routing table.
Detailed technical explanation
How to think about this question
iBGP requires that the next-hop of a route be reachable via an IGP or static route for the route to be installed in the routing table. In a site-to-site VPN, the next-hop is often the remote router's physical interface IP, which is not reachable through the IPsec tunnel unless the tunnel is configured as a virtual tunnel interface (VTI) or the crypto map includes the necessary ACL to encrypt traffic to that IP. Without reachability, BGP holds the route in its table but does not install it, which can be verified with 'show ip bgp' and 'show ip route'.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
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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 — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The next-hop address for the BGP routes is the physical interface IP of the remote router, which is not reachable through the tunnel, so the route is not installed. — In iBGP, the next-hop for routes learned from an eBGP peer is not changed by default. When the remote router advertises prefixes, it sets the next-hop to its physical interface IP address. If the IPsec tunnel is configured to encrypt traffic between the two routers' tunnel endpoints (e.g., virtual tunnel interfaces or crypto maps applied to physical interfaces), the physical interface IP of the remote router may not be reachable through the tunnel. BGP will not install a route in the routing table if the next-hop is not reachable via a valid route in the routing table, even if the BGP session is up and the prefixes are in the BGP table.
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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Last reviewed: Jun 24, 2026
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