For a GRE tunnel to be up between two routers, which of the following must be configured?

A.
Loopback Interface

B.
IP reachability between the loopback interfaces

C.
Dynamic Routing between routers.

D.
Tunnel interfaces must be in the same subnet.

Explanation:
http://www.cisco.com/en/US/tech/tk583/tk372/
technologies_configuration_example09186a008009438e.shtml
http://www.h3c.com/portal/Technical_Support___Documents/Technical_Documents/
Routers/H3C_SR8800_Series_Routers/Configuration/Operation_Manual/
H3C_SR8800_CG-Release3347-6W103/05/201202/739624_1285_0.htm