Answer: For IPSEC Site-to-Site VPN, allow ports UDP 500 IKE, UDP 4500 NAT-Traversal, and protocols ESP IP Protocol 50 and AH IP Protocol 51 on the firewall.

For IPSEC Site-to-Site VPN to function correctly through a firewall, certain ports and protocols must be permitted to ensure secure and reliable communication between the VPN endpoints. Here’s a detailed breakdown:

UDP Port 500

Used for the Internet Key Exchange (IKE) phase 1 negotiation process, allowing VPN gateways to establish a secure communication channel.

UDP Port 4500

Essential for NAT-Traversal (NAT-T), this port allows IPSEC traffic to pass through NAT devices by encapsulating IPSEC packets in UDP.

IP Protocol 50

Encapsulating Security Payload (ESP) provides confidentiality, data origin authentication, connectionless integrity, an anti-replay service, and limited traffic flow confidentiality.

IP Protocol 51

Authentication Header (AH) provides authentication, integrity, and anti-replay for IP packets, although it’s less commonly used compared to ESP.

Conclusion

To enable IPSEC Site-to-Site VPN through a firewall, it’s necessary to allow UDP ports 500 and 4500, along with IP protocols 50 (ESP) and 51 (AH). These settings ensure the secure and efficient operation of VPN connections, facilitating encrypted communication between sites.