A. The IBGP neighbor is not directly connected.
B. BGP synchronization is enabled in a transit autonomous system with fully - meshed IBGP neighbors.
C. The EBGP neighbor ebgp - multihop option is set to the de fault value.
D. The BGP update interval is different between the two BGP neighbors.
E. The BGP neighbor is referencing an incorrect autonomous system number in its neighbor statement.
F. There is an access list blocking all TCP traffic between the two  BGP neighbors.