Click the exhibit. Router R6 is a route reflector for clients R1, R2 and R5. Router R5 is configured with a local preference of 200. On which routers is "advertise-external" needed for router R6 to…

Question

Options

• AOn router R5 only.
• BOn either router R1or R2.
• COn both routers R1and R2.
• DOn all routers R1, R2 and R5.

• A
  6% (1)
• B
  71% (12)
• C
  6% (1)
• D
  18% (3)

Explanation

Option B is correct because R5 (local-pref 200) already advertises its best eBGP route to R6 without any special configuration - R6 already sees one route. Without advertise-external, R1 and R2 suppress their eBGP routes entirely when advertising to R6, because their best-selected path is R5's iBGP route (local-pref 200 wins path selection), and iBGP split-horizon prevents re-advertising it. Enabling advertise-external on either R1 or R2 causes that single router to also send its best external path to R6, resulting in exactly two routes: R5's + one external.

Why the distractors fail:

• A (R5 only): R5's external route is already its best path and is already sent to R6 - advertise-external on R5 is a no-op here; R6 still sees only one route.
• C (both R1 and R2): Both advertising their external routes gives R6 three distinct routes (R5 + R1 + R2), violating the "exactly two" constraint.
• D (all three): Same over-count issue as C; R5's configuration is redundant and R1+R2 together produce three routes.

Memory tip: The phrase "exactly two" is your anchor - R5 is already one route, so you need exactly one more. "Either R1 or R2" means pick one, not both. Whenever you see "exactly N routes" with a route reflector, count existing routes first, then determine how many advertise-external nodes fill the gap without exceeding it.

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