Refer to the exhibit. An engineer must prevent the R6 loopback from getting into Area 2 and Area 3 from Area 0. Which action must the engineer take?

Question

Exhibits

Options

• AApply a fitter list inbound on R2 and R9
• BApply a filter list outbound on R3 and R7
• CApply a filter list outbound on R7 only
• DApply a filter list inbound on R3 and R7

• A
  8% (5)
• B
  5% (3)
• C
  2% (1)
• D
  85% (50)

Explanation

OSPF Filter List Explanation

Option D is correct because in OSPF, a filter list applied inbound on an ABR filters LSAs being brought into that area from Area 0. R3 is the ABR between Area 0 and Area 2, and R7 is the ABR between Area 0 and Area 3; applying an inbound filter on both prevents the R6 loopback prefix from being advertised into those respective areas.

Why the distractors are wrong:

• Option A is incorrect because applying a filter inbound on R2 and R9 targets the wrong routers - these are not necessarily the ABRs controlling traffic into Areas 2 and 3, and filtering at R2 would be filtering into Area 0 rather than out of it.
• Option B is incorrect because an outbound filter on R3 and R7 would filter prefixes leaving those areas toward Area 0, not preventing Area 0 routes from entering Areas 2 and 3.
• Option C is incorrect because filtering on R7 only would block the prefix from Area 3 but leave Area 2 unprotected, requiring both ABRs to be configured.

Memory Tip: Think of OSPF filter lists from the ABR's perspective - inbound means "coming into my area from the backbone," so to block a backbone route from reaching a non-backbone area, always filter inbound on the receiving ABR.

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