What two features are benefits of using GRE tunnels with IPsec over using an IPsec tunnel alone in building-to-building site-to-site VPNs? (Choose two.)
A. allows dynamic routing securely over the tunnel
B. IKE keepalives are unidirectional and sent every ten seconds
C. reduces IPsec headers overhead since tunnel mode is used
D. supports non-IP traffic over the tunnel
E. uses Virtual Tunnel Interface (VTI) to simplify the IPsec VPN configuration
