Final answer:
The mode you must use with multiple VPNs, PSKs, SAs, same local gateway is Tunnel mode. It encapsulates the entire IP packet of one network within another network, allowing the original packet to traverse through the VPN tunnel.
Step-by-step explanation:
To connect multiple VPNs, PSKs, SAs with the same local gateway, you need to use Tunnel mode. Tunnel mode encapsulates the entire IP packet of one network within another network, allowing the original packet to traverse through the VPN tunnel. It is commonly used for site-to-site VPN connections, where entire networks are connected.
For example, let's say you have two remote offices connected to a central headquarters using VPN. Each remote office has its own VPN, PSKs (Pre-Shared Keys), and SAs (Security Associations). To securely transmit data between the remote offices and the headquarters, you would configure the VPNs to use Tunnel mode.
In Tunnel mode, the original IP packet is encrypted, encapsulated within a new IP packet, and sent to the remote network. At the receiving end, the outer IP packet is removed, and the original packet is decrypted and delivered to the intended destination.