- A
The static route to 10.10.10.0/24 via tunnel causes recursive routing failure because the tunnel destination is not reachable.
The static route references the tunnel interface, but the tunnel's destination IP must be reachable via another route. If not, the route is invalid, and BGP cannot use the next-hop.
- B
BGP next-hop-self is not configured on R2.
Why wrong: Next-hop-self would change the next-hop to R2's tunnel IP, but the issue is reachability of the next-hop, not its value.
- C
The IPsec tunnel is not encrypting BGP traffic.
Why wrong: BGP peering is established, so IPsec is working for BGP traffic.
- D
The BGP network statement for 10.10.10.0/24 is missing on R2.
Why wrong: R2 is advertising the route, as R1 sees it in BGP table.
Question 979 of 2,152
Static Route Recursive Routing Failure over IPsec Tunnel
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 are connected via an IPsec VPN tunnel. R1 has a static route to 10.10.10.0/24 pointing to the tunnel interface. R2 has a static route to 192.168.1.0/24 pointing to the tunnel interface. Both routers have BGP configured between loopback addresses over the tunnel. BGP peering is established, but R1 cannot ping 10.10.10.1 (R2's loopback) from its loopback. R1's show ip bgp shows the route as valid but not best. What is the root cause?
Quick Answer
The answer is recursive routing failure caused by an unreachable tunnel destination. This occurs because R1’s static route to 10.10.10.0/24 points to the tunnel interface, but the router cannot resolve the tunnel’s destination IP address—the remote peer’s public address—via any valid routing path. Without a reachable next-hop for the tunnel itself, the router cannot forward traffic into the IPsec VPN, so the BGP route remains valid but not best, and pings to 10.10.10.1 fail. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how static routes interact with overlay tunnels and the recursive lookup process; a common trap is assuming a tunnel interface is always “up/up” without verifying the underlying transport route. Remember the memory tip: “Tunnel up does not mean tunnel reachable—check the route to the peer’s public IP first.”
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 static route to 10.10.10.0/24 via tunnel causes recursive routing failure because the tunnel destination is not reachable.
The correct answer is A because R1's static route to 10.10.10.0/24 points to the tunnel interface, but the tunnel destination (the remote peer's public IP) is not reachable via a valid routing path. This creates a recursive routing failure: the router tries to resolve the tunnel interface's next-hop (the tunnel destination) but cannot find a route to it, causing the BGP route to be marked as valid but not best. Without a reachable tunnel destination, the IPsec VPN cannot forward traffic, so pings from R1's loopback to 10.10.10.1 fail.
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 static route to 10.10.10.0/24 via tunnel causes recursive routing failure because the tunnel destination is not reachable.
Why this is correct
The static route references the tunnel interface, but the tunnel's destination IP must be reachable via another route. If not, the route is invalid, and BGP cannot use the next-hop.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
BGP next-hop-self is not configured on R2.
Why it's wrong here
Next-hop-self would change the next-hop to R2's tunnel IP, but the issue is reachability of the next-hop, not its value.
- ✗
The IPsec tunnel is not encrypting BGP traffic.
- ✗
The BGP network statement for 10.10.10.0/24 is missing on R2.
Why it's wrong here
R2 is advertising the route, as R1 sees it in BGP table.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept of recursive routing failure in IPsec VPN scenarios, where candidates mistakenly focus on BGP configuration issues (like next-hop-self or network statements) instead of recognizing that the tunnel destination must be reachable independently of the tunnel itself.
Detailed technical explanation
How to think about this question
Recursive routing failure occurs when a router's route to a destination depends on a next-hop that itself requires the same interface to be reachable, creating a loop. In IPsec VPNs, the tunnel interface is a virtual interface whose destination (the remote peer's public IP) must be reachable via a physical interface or a separate route; if that route is missing or points back to the tunnel, the route is considered invalid or not best. This is a common misconfiguration when using static routes over tunnel interfaces without ensuring the tunnel endpoint is reachable via an underlying routing table entry.
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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Visual reference
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
What to study next
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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 static route to 10.10.10.0/24 via tunnel causes recursive routing failure because the tunnel destination is not reachable. — The correct answer is A because R1's static route to 10.10.10.0/24 points to the tunnel interface, but the tunnel destination (the remote peer's public IP) is not reachable via a valid routing path. This creates a recursive routing failure: the router tries to resolve the tunnel interface's next-hop (the tunnel destination) but cannot find a route to it, causing the BGP route to be marked as valid but not best. Without a reachable tunnel destination, the IPsec VPN cannot forward traffic, so pings from R1's loopback to 10.10.10.1 fail.
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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Last reviewed: Jun 24, 2026
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