Which of these BGP paths will be the preferred one ?

The BGP best path selection algorithm is a deterministic, step-by-step process. The Local Preference attribute is evaluated very early in this sequence, specifically after the non-standard Weight attribute. A path with a higher Local Preference value is always preferred over a path with a lower value, regardless of other attributes like AS Path length, MED, or whether the path was learned via eBGP or iBGP. Because Local Preference is checked before the other attributes listed in the options, it is the most influential and will be the deciding factor among the choices provided.

A. The preference for an eBGP path over an iBGP path is a valid step, but it occurs much later in the selection algorithm, after Local Preference, AS Path, and MED have already been compared.

B. The shortest AS Path is a fundamental BGP metric, but it is only evaluated if the competing paths have an identical, and highest, Local Preference value.

C. The Multi-Exit Discriminator (MED) is considered even after Local Preference and AS Path length. It is used to influence an external neighbor's routing decision into the local AS.

1. Fortinet FortiOS 7.4.0 Administration Guide: In the "Networking" section

under "BGP

" the "BGP path selection" subsection explicitly lists the order of attributes. It shows Local Preference (Step 2) is evaluated before AS path (Step 4)

MED (Step 6)

and preferring eBGP over iBGP paths (Step 7).

Reference: FortiOS Administration Guide 7.4.0

"Networking > BGP > BGP path selection"

Page 1035.

2. RFC 4271 - A Border Gateway Protocol 4 (BGP-4): This is the foundational IETF standard for BGP. Section 9.1.2.2

"Decision Process

" outlines the path selection rules.

Reference: IETF RFC 4271

Section 9.1.2.2

"Decision Process". Phase 2

step (c) describes selecting the path with the highest LOCALPREF. This step precedes the evaluation of ASPATH length (step f)

MED (step g)

and eBGP over iBGP (step h). (DOI: https://doi.org/10.17487/RFC4271)