Which IPsec mode will encrypt a GRE tunnels and reduced overhead?
B. multipoint GRE
When IPSec is running in tunnel mode, a new IPv4 header will be created, followed by the AH and/or ESP header(s), then the original IP header. In transport mode, there is no new IPv4 header, the AH and/ESP header(s) are added above the original IP header. Therefore there is reduced overhead of adding an additional IP header.
CCNP Routing and Switching