A network designer has provisioned a router to use IPsec to encrypt the traffic over a GRE tunnel going to a web server at a remote location. From the router, the network designer can ping the web server, although the users in the office comment that they are unable to reach it. (Note: The DF bit is not set.) Which aspect should be changed in the design of the virtual connection? 

Answer options:

A. IP addresses of the GRE tunnel endpoints
B. IPsec configuration
C. MTU size on the GRE tunnel
D. encapsulation of the GRE tunnel

C