VPN Technology question
In IKEv1 Phase1, since Diffie-Hellman is the actual algorithm that peers are using to securely negotiate the actuall encryption and decryption keys and as known in order for DH to work, there must be an agreed Shared key between peers so based on this agreed shared key, peers can negotiate and come up with the final secret key.
My question is: Is the DH agreed shared key used initally by peers is the same key as IKEv1 Phase1 Authentication PSK, since this is the only Shared key configured locally in both peers? And if it's not the PSK used for Authentication, how DH works to create a secure channel so encryption and decryption key can be negotiated?
I actually said that "it is known that in order for DH to work, there must be an agreed Shared key between peers" is based on what I understood from this figure that shows how DH key exchange is calculated.