300-410 VPN Technologies Practice Question
A network administrator is deploying a GET VPN using Cisco IOS routers. The key server is configured with a cooperative key server (COOP) for redundancy. The administrator notices that some group members are not registering with the primary key server. Which protocol and port must be allowed through the firewall for the group members to register with the key server?
⚠ Common exam trap
The trap here is assuming GET VPN uses standard IPsec ports like UDP 500 or 4500 for registration, when it actually uses GDOI on UDP 848.
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
✓
GDOI on UDP port 848
GET VPN group members use GDOI on UDP port 848 to register with the key server. This port must be permitted through firewalls to allow registration and key retrieval.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
GDOI on UDP port 848
Why this is correct
GET VPN group members use the GDOI protocol to register with the key server. GDOI operates on UDP port 848. Allowing this port through the firewall is essential for group members to register and receive keys and policies from the key server.
- ✗
ISAKMP on UDP port 500
Why it's wrong here
ISAKMP on UDP 500 is used for IKE Phase 1 and Phase 2 in traditional IPsec VPNs. GET VPN group members do not use IKE to register with the key server; they use the Group Domain of Interpretation (GDOI) protocol, which uses a different port.
- ✗
IPsec ESP on IP protocol 50
Why it's wrong here
IPsec ESP (protocol 50) is used to encrypt data traffic between group members, not for registration. Registration with the key server uses GDOI, which is a separate protocol. Blocking ESP would prevent data encryption but not registration.
- ✗
NAT-T on UDP port 4500
Why it's wrong here
NAT-T on UDP 4500 is used for IPsec traffic traversal through NAT devices. GET VPN does not use NAT-T for registration; it uses GDOI on UDP 848. NAT-T is relevant for traditional IPsec VPNs when NAT is in the path.
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
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