简答题

R1，R2，R3 are all in same ospf area， there is a virtual link between R1 and R3. Pls answer： Are the hello packets exchanged between the virtual link？

A.upon a port state change
B.upon receiving a topology change notification
C.when transitioning from discarding to forwarding
D.when transitioning from forwarding to discarding
E.only when changing from listening to discarding
F.when CAM resources have been completely used up

A.it is newer
B.it has smaller timers
C.it has less overhead
D.it is not timer-based