What are the three requirements for an eBGP neighborship to form?

• A. Different AS number, define neighbors (TCP session) and reachability (IP address used in the neighbor command must be reachable)
• B. Same AS number, define neighbors (TCP session) and reachability
• C. Different AS number, no TCP session required, and reachability
• D. Different AS number only

Answer: (A)

In eBGP, adjacency is formed when two routers in different autonomous systems can establish a TCP session and agree on who they are talking to. The three requirements reflect how BGP conversations actually happen between distinct networks: the remote AS number must be different from your local AS, you must explicitly define the neighbor so a TCP connection can be established, and the neighbor’s IP address must be reachable so that the TCP session can be opened.

Why this set fits: using different AS numbers is what distinguishes eBGP from iBGP, since eBGP is designed for route exchange between separate networks. A neighbor must be defined so the router knows where to try to establish the TCP session and exchange BGP messages. And reachability ensures that the neighbor’s IP can be reached over the IGP/static routes so the TCP connection can actually be established and sustained.

Why the other possibilities don’t fit: having the same AS number would imply an iBGP session, not eBGP. Not having a TCP session would prevent any BGP message exchange, so no neighborship forms. And having only reachability or only a different AS number without configuring the neighbor would leave you without either the necessary TCP session or the explicit neighbor definition required for eBGP to come up.