hardMultiple ChoiceObjective-mapped
300-410 Practice Question: In Cisco IOS, what is the default encryption…
In Cisco IOS, what is the default encryption algorithm for IKEv1 phase 1 if not specified in the ISAKMP policy?
⚠ Common exam trap
Cisco often tests the default encryption algorithm for IKEv1 phase 1, and the trap here is that candidates assume a stronger algorithm like AES or 3DES is the default, but Cisco IOS defaults to the weaker DES for backward compatibility.
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
✓
DES
In Cisco IOS, when an IKEv1 ISAKMP policy is configured without specifying an encryption algorithm, the default encryption algorithm is DES (Data Encryption Standard). This is because Cisco IOS defaults to DES for IKEv1 phase 1 if no encryption is explicitly defined in the ISAKMP policy, as per the default policy parameters. DES uses a 56-bit key and is considered weak by modern standards, but it remains the default for backward compatibility.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
AES 256
Why it's wrong here
AES 256 is not the default; it must be explicitly configured.
- ✗
3DES
Why it's wrong here
3DES is not the default; DES is.
- ✓
DES
Why this is correct
Cisco IOS defaults to DES if no encryption is specified.
- ✗
AES 128
Why it's wrong here
AES 128 is not the default.
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 by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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