Which BGP attribute is used to select the best path when multiple paths exist to the same destination?

While multiple attributes are involved in the BGP path selection algorithm, Local Preference is the primary attribute used within an Autonomous System (AS) to express a preference for a specific exit path. When an AS has multiple exit points to the internet or another AS, an administrator sets a higher Local Preference value on the routes learned from the desired exit point. This attribute is evaluated before the AS Path length, making it the most influential tool for outbound traffic engineering within one's own AS.

A. MED is used to influence how an external AS sends traffic into your AS, not for selecting your own exit path.

C. AS Path length is a factor, but Local Preference is evaluated first and is the standard attribute for expressing path preference.

D. The Next Hop attribute simply identifies the IP address of the next router; it is not a metric used for path comparison.

- Juniper Networks TechLibrary

"BGP Path Selection

" section "BGP Path Selection Process." This document lists the BGP decision process

showing that "Highest local preference" (Step 2) is evaluated before "Shortest AS path" (Step 4).

- RFC 4271

"A Border Gateway Protocol 4 (BGP-4)

" Section 9.1.2

"Phase 2: Route Resolution." The decision process is outlined

stating

"The LOCALPREF attribute is used by a BGP speaker to inform its other internal peers of the degree of preference it has for an advertised route."