300-410 VPN Technologies Practice Question
A network engineer is troubleshooting an IPsec site-to-site VPN between two Cisco IOS routers. Phase 1 is up, but Phase 2 fails to establish. The engineer suspects a mismatch in the IPsec proposal. Which command would show the configured IPsec transform set and the algorithms being used?
⚠ Common exam trap
The trap here is assuming that show crypto ipsec sa will show the configured transform set; it only shows established SAs, which may be absent when Phase 2 fails.
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
✓
show crypto ipsec transform-set
When Phase 2 fails, the most likely cause is a mismatch in the IPsec transform set. The show crypto ipsec transform-set command reveals the configured algorithms for each transform set, enabling comparison between peers. While other commands show SA status or map references, only this command directly displays the transform set details needed to identify the mismatch.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
show crypto map
Why it's wrong here
The show crypto map command displays the crypto map configuration, including the peer, ACL, and the transform set name referenced. While it shows which transform set is used, it does not show the algorithms within that transform set. To see the actual algorithms, the engineer must use show crypto ipsec transform-set. Therefore, this command alone is insufficient to diagnose a proposal mismatch.
- ✗
show crypto isakmp sa
Why it's wrong here
The show crypto isakmp sa command displays the status of Phase 1 IKE security associations, including the peer IP, state, and encryption/hash algorithms used for IKE. It does not show the IPsec transform set or Phase 2 algorithms. While useful for verifying Phase 1, it cannot confirm the IPsec proposal settings that are causing Phase 2 failure.
- ✗
show crypto ipsec sa
Why it's wrong here
The show crypto ipsec sa command shows the status of Phase 2 IPsec security associations, including the encapsulating SPI, transform set used, and packet counters. However, it only displays SAs that have been successfully established. If Phase 2 fails, this command may show no SAs or incomplete information, and it does not directly reveal the configured transform set parameters before they are applied.
- ✓
show crypto ipsec transform-set
Why this is correct
The show crypto ipsec transform-set command lists all configured IPsec transform sets, including the name and the specific protocols and algorithms (e.g., esp-aes, esp-sha-hmac). This allows the engineer to compare the transform set on both routers and identify any mismatch in encryption or authentication algorithms that would cause Phase 2 negotiation to fail. It is the most direct way to verify the configured IPsec proposal.
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.
Go deeper
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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.