The BGP path selection process within an Autonomous System (AS) is primarily influenced by BGP attributes. The LOCALPREF attribute is specifically designed for this purpose. It is a well-known, discretionary attribute that is exchanged only between internal BGP (iBGP) peers. By setting a higher LOCALPREF value for routes learned from a specific external peer, an administrator can signal to all other routers within the same AS that this is the preferred exit point for that destination. A higher LOCALPREF value is always preferred, directly influencing the outbound routing policy for the entire AS.

A. BGP uses the ASPATH length, not the number of router hops (an IGP metric like in RIP), for its path selection process.

C. The link metric (e.g., cost) is a value used by Interior Gateway Protocols (IGPs) like OSPF, not by BGP for its primary path selection.

D. Administrative Distance (AD) is used by a router to choose between different routing protocols (e.g., BGP vs. OSPF), not to influence the BGP path selection algorithm itself.

1. RFC 4271: A Border Gateway Protocol 4 (BGP-4)

Y. Rekhter

T. Li

S. Hares

Eds.

January 2006.

Section 9.1.2.2

"Decision Process": This section details the BGP best-path selection algorithm. Step (c) explicitly states: "The route with the highest local preference is preferred." This confirms that LOCALPREF is a primary factor in the decision process. The attribute is defined in Section 5.1.4 as being used "by a BGP speaker to inform its other internal peers of the advertising speaker's degree of preference for an advertised route."

2. Kurose

J. F.

& Ross

K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson.

Chapter 5.3

"The Border Gateway Protocol (BGP) in the Internet": This chapter explains BGP route selection policies. It describes how Local Preference is used to select a specific gateway router for a destination among multiple options within an AS

stating

"The local preference attribute is used to indicate a preference for one route over another... and is thus an AS-internal-only BGP attribute."

3. MIT OpenCourseWare

Course 6.829

Computer Networks

Fall 2002.

Lecture 14

"BGP": The lecture notes describe the BGP decision process. They list the attributes in order of evaluation

placing "Highest Local Preference" as the first major policy-based step after basic reachability checks

reinforcing its role in influencing route selection within an AS. (Available via MIT OCW website).