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.
Answer: A E
Explanation
An underlying connection between two BGP speakers must be established before any routing information is exchanged. This connection takes place on TCP port 179 so if an access list blocks all TCP traffic between the two BGP neighbors, BGP neighbor relationship can not be established -> A is correct.
The IBGP neighbors don’t need to be directly connected -> B is not correct.
BGP synchronization only prevents routes sent to other EBGP neighbors before that route exists in the routing table. It doesn’t prevent BGP neighbor relationship -> C is not correct.
After the first initial exchange (which exchanges routes and synchronize their tables), a BGP speaker will only send further updates upon a change in the network topology -> BGP does not have a fixed update interval -> D is not correct.
BGP neighbor relationship is established when both ends (routers) are manually configured with the “neighbor neighbor-IP remote-as neighbor-AS” command on both sides of the connection. If the neighbor-AS is wrong, the neighbor relationship can not be established -> E is correct.
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