A router in AS 65100 receives two BGP routes. The first route is for 138.1.1.0/24 with an AS Path of 65200 65000, local Preference of 100, MED of 100, and router ID 10.10.1.10. The second route is…

Question

Options

• AThe first BGP route.
• BThe second BGP route.
• CBoth routes are used.
• DIt is impossible to determine without the value of the origin code for both routes.

• A
  80% (40)
• B
  12% (6)
• C
  2% (1)
• D
  6% (3)

Explanation

Route 1 (138.1.1.0/24) wins because of longest prefix match - a fundamental IP forwarding rule that takes precedence over all BGP path selection attributes. The destination 138.1.1.7 matches both prefixes, but /24 is more specific than /16, so the router selects it immediately without ever comparing Local Preference, MED, or Router ID.

Why the distractors are wrong:

• B is tempting because Route 2 has higher Local Preference (150 vs 100), but Local Preference is only compared between routes to the same destination prefix - these are different prefixes, so BGP attributes are irrelevant here.
• C is wrong because a router forwards a packet out a single best-path interface, not both simultaneously.
• D is wrong because origin code is a BGP tiebreaker that would only matter if you were choosing between two routes for the same prefix; longest prefix match resolves this before any BGP attribute comparison begins.

Memory tip: Think "most specific address wins first" - the router finds the longest matching prefix before it ever opens the BGP attribute rulebook.

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