A primary reason to use BGP in an AS

• A. To manipulate routing policy and route selection for traffic entering and leaving the AS
• B. To avoid any routing policy changes
• C. To replace DNS
• D. To provide host-level firewalling

Answer: (A)

BGP in an AS is used to control how traffic is routed to and from networks outside the AS by applying routing policies. Operators exchange reachability with neighboring ASes and use BGP attributes (like LOCAL_PREF to choose preferred exit points, AS_PATH to influence path selection, and MED to influence incoming choices) to shape both outbound and inbound traffic. This enables practical traffic engineering—selecting which upstream provider to use, balancing load, and implementing backup paths—while controlling which routes are accepted or advertised. The other options describe DNS replacement or host-level firewalling, which are outside BGP’s scope, and the idea of avoiding routing policy changes runs counter to what BGP is designed to do: implement and enforce routing policies.