mediumMultiple Choice
300-410 Practice Question: Consider the following configuration on router…
Consider the following configuration on router R1:
crypto isakmp policy 10
encryption aes 256 authentication pre-share group 14 lifetime 86400 !
crypto isakmp key cisco123 address 192.168.1.2
!
crypto ipsec transform-set TSET esp-aes 256 esp-sha-hmac
mode tunnel !
crypto map CMAP 10 ipsec-isakmp
set peer 192.168.1.2 set transform-set TSET match address 101 !
interface GigabitEthernet0/1 ip address 192.168.1.1 255.255.255.0 crypto map CMAP
!
access-list 101 permit ip 10.1.1.0 0.0.0.255 10.2.2.0 0.0.0.255
If the remote peer has an ISAKMP policy with encryption 3des, what will happen?
⚠ Common exam trap
Cisco often tests the misconception that IKE will automatically negotiate or fall back to a weaker algorithm, but in reality, IKE phase 1 requires an exact match of all policy parameters, and a mismatch causes the entire VPN to fail.
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
✓
IKE phase 1 will fail due to encryption algorithm mismatch.
IKE phase 1 requires both peers to have a matching ISAKMP policy, including the encryption algorithm. Since R1 is configured with AES 256 and the remote peer uses 3DES, there is no common proposal, causing phase 1 to fail. Cisco IOS does not automatically negotiate or fall back to a different encryption algorithm; the mismatch results in a failed IKE SA.
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 tunnel will use 3DES because the remote peer's policy is accepted.
Why it's wrong here
The local policy offers only AES 256, so the peer's 3DES proposal finds no match and phase 1 fails; the tunnel cannot adopt 3DES. It is tempting because IKE negotiates a mutually acceptable policy, but negotiation picks from offered proposals rather than accepting a peer's unconfigured encryption value.
- ✓
IKE phase 1 will fail due to encryption algorithm mismatch.
Why this is correct
IKE phase 1 requires both peers to agree on the encryption algorithm within their ISAKMP policies. R1 offers aes 256 while the remote peer offers 3des only, so no matching proposal exists and main mode negotiation fails.
- ✗
The router will automatically adjust to use 3DES.
Why it's wrong here
ISAKMP policy negotiation requires an exact match of encryption, hash, authentication, and Diffie-Hellman group; with no common encryption proposal, phase 1 fails and no tunnel forms. It is tempting because IKE does negotiate parameters, but that negotiation selects from mutually offered proposals rather than downgrading one peer's configured AES 256 to 3DES.
- ✗
The tunnel will establish but use AES 256 anyway.
Why it's wrong here
Phase 1 fails because the peers share no common encryption proposal, so no security association is built and no tunnel carries traffic, AES 256 or otherwise. It is tempting because transform sets define IPsec encryption, but those parameters are only reached after successful ISAKMP negotiation, which never completes here.
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 by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.