A router in AS 65100 receives three routes for 138.120.0/16. 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-In.
• BOnly the second route will appear in the RIB-In.
• COnly the third route will appear in the RIB-In.
• DAll routes will appear in the RIB-In.

• A
  4% (1)
• C
  8% (2)
• D
  88% (21)

Explanation

All three routes appear in the RIB-In because the RIB-In is a pre-decision store that holds every route received from BGP peers, before any attribute-based selection occurs - it is essentially BGP's inbox. Local preference, AS path length, and other attributes are only evaluated during the BGP best-path decision process, which runs after routes are stored in the RIB-In to determine which route enters the Loc-RIB and eventually the forwarding table. Options A, B, and C are wrong because they confuse RIB-In with best-path selection: if the question had asked which route is installed or preferred, then local preference (200 wins) would make the third route the answer - but that is a different question entirely. The memory tip: think of RIB-In as "receive all, decide later" - it never filters, it just collects; filtering and selection happen downstream in the Loc-RIB.

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