A system administrator is configuring a site-to-site VPN tunnel. Which of the following should be configured on the VPN concentrator during the IKE phase?

RIPEMD is a cryptographic hash function used for data integrity verification, not a key exchange mechanism configured during the IKE phase of VPN establishment.

ECDHE (Elliptic Curve Diffie-Hellman Ephemeral) is a variant of Diffie-Hellman that provides forward secrecy and is used in TLS, but it is not the standard configurable key exchange group referenced in the context of IKE phase configuration on VPN concentrators.

Diffie-Hellman (DH) is the foundational key exchange protocol used during IKE Phase 1 to allow two VPN peers to establish a shared secret over an untrusted network without transmitting the secret itself. The administrator must configure a DH group (e.g., Group 2, 14, 19) on the VPN concentrator, which determines the strength of the key exchange. This shared secret is subsequently used to derive the symmetric keys that protect the IKE SA (Security Association) and ultimately the IPsec tunnel.

HTTPS is an application-layer protocol for securing web traffic using TLS and is entirely unrelated to the IKE phase of an IPsec site-to-site VPN tunnel configuration.