Which encryption mode is used when a packet is sent from a site-to-site VPN connection where the source and destination IP address portion of a packet is unencrypted?

PPTP (Point-to-Point Tunneling Protocol) is an older, less secure VPN protocol, and its encryption mode is not referred to as 'Transport' in this context.

Secure Shell (SSH) is a protocol for secure remote access and data transfer, not typically an encryption mode for site-to-site VPNs in the IPSec context.

In IPSec Transport mode, only the payload of the IP packet is encrypted and/or authenticated, meaning the original IP header, including the source and destination IP addresses, remains in plain text and visible for routing purposes. This differs from Tunnel mode, which encapsulates and encrypts the entire original IP packet within a new IP packet.

PPPoE (Point-to-Point Protocol over Ethernet) is used for encapsulating PPP frames over Ethernet networks, often for DSL connections, and is not an encryption mode for VPNs.