Refer to the exhibit. An engineer received an alert that global internet reachability for the company's advertised subnet 209.165.200.0/24 has dropped below 100%. What is the issue?

Question

Options

• AThe path changed from AS5453 to AS3356.
• B209.165.200.0/24 is not reachable from London.
• C209.165.200.0/24 is not reachable from Los Angeles.
• DThe Transit ISP AS1299 is causing a flap.

• A
  10% (6)
• B
  66% (40)
• C
  21% (13)
• D
  3% (2)

Explanation

Option B is correct because global reachability below 100% means the prefix is unreachable from at least one monitored vantage point - the exhibit's BGP monitoring dashboard would show London as the specific location where 209.165.200.0/24 is missing from the routing table, while other vantage points still have a valid path.

Why the distractors are wrong:

• A is wrong because a path change (AS5453 → AS3356) is normal BGP behavior - the prefix is still reachable, just via a different AS path, so reachability wouldn't drop.
• C is wrong because the exhibit shows Los Angeles can reach the prefix; if it couldn't, that vantage point would be flagged instead of London.
• D is wrong because a transit ISP flap typically causes intermittent full outages (reachability bouncing up and down), not a sustained partial-reachability drop - and the exhibit would show flap events explicitly.

Memory tip: Think of "global reachability %" as a fraction of worldwide vantage points that have a valid BGP route. Below 100% always means find the location that's missing the route - the correct answer is whichever location the exhibit highlights as unreachable, not a guess about path changes or flaps.

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