A router in AS 65100 receives three routes for 138.120.0.0/16. The first route has an AS Path of 65200 65400 65300 and a local preference of 150. The second route has an AS Path of 65200 65300 and a local preference of 100. The third route has an AS Path of 65600 65300 and a local preference of 100. Which route will appear in the RIB-out?

The BGP best-path selection algorithm evaluates attributes in a specific order. The very first attribute considered for routes from iBGP peers is LOCALPREF, where a higher value is preferred. The first route has a LOCALPREF of 150, while the other two have a LOCALPREF of 100. Therefore, the first route is unequivocally selected as the best path. By default, only the single best path for a prefix is installed in the routing table (RIB) and advertised to other BGP peers (placed in the RIB-out).

B. The second route has a lower local preference (100) than the first route (150) and will not be selected as the best path.
C. The third route also has a lower local preference (100) than the first route (150) and will not be selected as the best path.
D. BGP selects only one best path per prefix to install in the RIB and advertise by default; it does not forward all received routes.

1. Rekhter, Y., Li, T., & Hares, S. (2006). RFC 4271 - A Border Gateway Protocol 4 (BGP-4). IETF. Section 9.1.2, "LOCALPREF," and Section 9.1, "Decision Process".

2. Nokia (2021). 7750 SR OS Router Configuration Guide, Release 21.10.R1. Document ID: 3HE 17279 AAAA TQZZA. "BGP Path Selection" section.

3. Doyle, J., & Carroll, J. (2006). Routing TCP/IP, Volume II. Cisco Press. Chapter 5, "BGP Path Attributes and the BGP Decision Process".