When to use BGP?

• A. The AS acts as a core transit network with minimal peers to reduce overhead
• B. The AS is small with no need to manipulate routing policy
• C. The AS has a single connection to the internet
• D. The AS allows packets to transit through it to reach other AS (a service provider), the AS has multiple connections to other AS, or routing policy and route selection for traffic entering and leaving the AS must be manipulated.

Answer: (D)

BGP is used when an AS needs control over how traffic enters and leaves across multiple external networks, and especially when it has a transit role or multiple connections to other autonomous systems. When an AS is not just a single-homed edge but a network that should allow other networks to transit through it, or when it has several connections to different providers, you need BGP to manage routing policy and path selection. That includes deciding which upstream to send outbound traffic to, which paths to prefer, and how to advertise your prefixes to peers.

If the AS has only one internet connection, or is small with no need to manipulate routing decisions, BGP adds unnecessary complexity. In such cases, you can rely on the provider’s default routes or use simpler interior routing protocols inside the network. But when you’re enabling transit for others, multihoming with multiple external connections, or you need to apply specific routing policies (like preferring one path over another, or controlling which routes enter or leave the AS), BGP provides the scalable, policy-driven inter-domain routing needed.