A.GRE can use transport mode and save up to twenty bytes per packet.
B.The corporate network includes many discontinuous blocks of IP networks and requires split tunneling.
C.The corporation's primary application is based on IP multicast.
D.Without GRE, there is no means to detect a head-end failure.
E.GRE would enable path MTU discovery.

A.Stateful Inspection Firewall
B.Intrusion Detection System
C.Spam Filtering 
D.802.1-based Authentication
E.Proxy Authentication
F.URL Filtering