What is Local Preference (LOCAL_PREF) and how can it influence security-related routing decisions?

• A. External attribute used by all peers to choose routes.
• B. Internal attribute to prefer certain exit points; use to avoid untrustworthy peers or links.
• C. Attribute that governs MED across multiple providers.
• D. Deprecated attribute with no routing impact.

Answer: (B)

Local Preference is an internal BGP path attribute used to decide which exit point your AS should use when sending traffic to a destination. Routers within the same autonomous system compare the LOCAL_PREF values for candidate routes and pick the one with the highest value. Because it is local to the AS, this attribute isn’t advertised to external neighbors—it's a tool for controlling outbound routing from inside your network.

This makes Local Preference a practical lever for security-related routing decisions. By assigning higher LOCAL_PREF to routes that go through trusted, monitored, or reliable exit points, you push traffic to leave your AS via those preferred paths. Conversely, you can lower the value for routes that traverse untrustworthy or less secure links, effectively steering traffic away from them. In short, it helps you avoid paths that could be compromised or unreliable, reducing exposure to interception or manipulation of outbound traffic.

It’s important to distinguish this from other attributes: Local Preference is internal to the AS and not shared with other ASes, whereas features like MED are used to influence inbound traffic and involve external neighbors. And Local Preference is not a deprecated feature; it remains a standard tool for routing policy and security-focused control of egress paths.