In BGP, what do LOCAL_PREF and MED influence?

• A. LOCAL_PREF controls neighbor authentication; MED controls route summarization
• B. LOCAL_PREF influences outbound path preference within an AS; MED indicates preferred entry points into an AS
• C. LOCAL_PREF indicates speed; MED indicates security level
• D. LOCAL_PREF and MED are not used in modern BGP

Answer: (B)

In BGP, how routes are chosen depends on where the decision happens: inside your own AS and at the points where traffic enters or leaves the AS. Local preference is the knob you turn to pick which outbound path to use for a destination within your AS. A higher local preference value means that route is preferred when your routers decide which path to send traffic along, and this attribute is not shared with other autonomous systems.

MED, the Multi-Exit Discriminator, serves as a hint to neighboring ASes about which entry point into your AS you prefer for inbound traffic. Lower MED values are favored, guiding external routers on which ingress point to use into your network.

So, local preference shapes outbound path choice inside your AS, while MED guides inbound entry points from other ASes. Other options mix up concepts like authentication, summarization, speed, or security, which aren’t what these attributes control, and these attributes are still in use in modern BGP.