Which aspect should be changed in the design of the virtual connection?

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

Answer: C