300-410 VPN Technologies Practice Question
A network administrator is troubleshooting an IPsec site-to-site VPN between two Cisco routers. The tunnel is up, but traffic is not passing. The administrator runs show crypto ipsec sa and notices that the inbound and outbound ESP SAs are present, but the packet counters are not incrementing. The ACL used for the crypto map is permit ip 10.1.1.0 0.0.0.255 10.2.2.0 0.0.0.255. Which action is most likely to resolve the issue?
⚠ Common exam trap
The trap here is assuming that an ACL mismatch is the culprit because it's a common VPN issue, but with SAs established, the problem is more likely routing or interesting traffic not being matched.
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
✓
Check the routing table to ensure that the remote protected subnet is reachable via the tunnel interface or that the next hop is correct.
When IPsec SAs are up but packet counters remain at zero, the router is not identifying or routing traffic into the VPN. The most common cause is a routing issue: the remote subnet is not reachable via the tunnel, or a more specific route directs traffic elsewhere. Verifying the routing table and ensuring the tunnel interface is the next hop for the protected subnet resolves the problem.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Confirm that the transform set matches on both peers.
Why it's wrong here
A transform set mismatch would prevent IPsec SAs from being established. Since the SAs are present, the transform sets must already match. Therefore, checking the transform set is not the cause of the packet counters not incrementing. The issue lies elsewhere, likely in routing or interesting traffic selection.
- ✗
Ensure that the preshared key is identical on both peers.
Why it's wrong here
A preshared key mismatch would cause IKE phase 1 to fail, and IPsec SAs would not be established. The presence of IPsec SAs indicates that IKE authentication succeeded. Thus, the preshared key is not the problem. The focus should be on why traffic is not being encrypted and sent through the tunnel.
- ✓
Check the routing table to ensure that the remote protected subnet is reachable via the tunnel interface or that the next hop is correct.
Why this is correct
If the IPsec SAs are established but packet counters are not incrementing, the router is not forwarding interesting traffic into the tunnel. This often occurs when the route to the remote protected subnet points out of the physical interface instead of the tunnel interface, or when there is no route at all. Correcting the routing ensures that packets matching the crypto ACL are sent through the VPN.
- ✗
Verify that the crypto ACL on the remote peer is a mirror image of the local ACL.
Why it's wrong here
The remote peer's crypto ACL should be a mirror image (source and destination reversed) for symmetric traffic. However, if the SAs are up and packet counters are not incrementing, the issue is likely that traffic is not being routed into the tunnel, not an ACL mismatch. An ACL mismatch would typically prevent SA establishment or cause only one-way traffic.
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.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.