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…

Question

Options

• AOnly the first route will appear in the RIB-Out.
• BOnly the second route will appear in the RIB-Out.
• COnly the third route will appear in the RIB-Out.
• DAll routes will appear in the RIB-Out.

• A
  82% (27)
• B
  6% (2)
• C
  3% (1)
• D
  9% (3)

Explanation

Option A is correct because Local Preference is evaluated before AS Path length in BGP's best-path selection algorithm. Route 1's local preference of 150 is higher than Routes 2 and 3 (both 100), making it the clear winner regardless of its longer AS Path (3 hops vs. 2). Only the single best-path route is placed into the RIB-out and advertised to peers.

Route B fails because even though Route 2 has a shorter AS Path (65200 65300), its local preference of 100 is outranked by Route 1's 150 - AS Path length is a tiebreaker that never gets evaluated here. Route C fails for the same reason - Route 3 also has local preference 100, and even a head-to-head comparison with Route 2 (same length) would require further tiebreakers. Option D fails because BGP selects exactly one best path per prefix for the RIB-out; all received routes sit in the Adj-RIB-In, but only the winner is advertised.

Memory tip: Use the acronym "WLLOM..." (Weight → Local Pref → Locally originated → AS Path length → Origin...) to recall BGP's decision order - Local Preference comes second (first in iBGP scenarios without Cisco Weight), so always check it before counting hops.

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