Which two conditions can cause BGP neighbor establishment to fail?

Which two conditions can cause BGP neighbor establishment to fail? (Choose two.)

A.
There is an access list blocking all TCP traffic between the two BGP neighbors.

B.
The IBGP neighbor is not directly connected.

C.
BGP synchronization is enabled in a transit autonomous system with fully-meshed IBGP
neighbors.

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.

Explanation:
BGP uses TCP port 179 to establish and maintain neighbor relationships, so any access lists or
firewalls must permit this port for BGP to function.
By default, EBGP multi-hop is not enabled, so the EBGP peer must be directly connected in order
for the local BGP router to know how to reach the EBGP peer.
BGP uses neighbor statements that specify the AS number of the BGP peer. If it is the same
number as the local BGP router process, then the BGP router knows that IBGP is used. If the AS

number for the specified peer is different, then EBGP is used. Either way, the specified neighbor
must be configured correctly, or the BGP peers will not become neighbors.