A. DR and BDR election errors between R1 and R2.
B. The R2 router has not received the OSPF hello packets from the R1 router.
C. Mismatched interface ma ximum transmission unit (MTU) configuration between the R1 and R2.
D. Mismatched OSPF hello interval configuration between the R1 and R2
E. Corrupted LSAs exchanges between the R1 and R2.