Refer to the exhibit. Networks 172.16.31.0/24 and 172.16.30.0/24 are advertised in area 34, and network 172.22.10.0/24 is advertised in area 0. A recent security review discovered that users…

Question

Exhibit

Options

• AApply route filtering on routers R3 and R4.
• BApply route filtering on router R3 only.
• CApply route filtering on routers R1 and R2
• DApply route filtering on router R4 only.

• A
  13% (8)
• B
  25% (15)
• C
  7% (4)
• D
  56% (34)

Explanation

Applying the filter on R4 alone is correct because R4 is the sole ABR (Area Border Router) connecting Area 34 to Area 0. Since inter-area routes must pass through the ABR to reach Area 0, filtering 172.16.31.0/24 on R4's outbound advertisements completely prevents R1 and R2 from ever learning the route - stopping unauthorized access at the source.

Why the distractors are wrong:

• A (R3 and R4): R3 is an internal router within Area 34, not an ABR into Area 0, so filtering there has no effect on what R1 and R2 receive. Filtering on R4 alone is sufficient.
• B (R3 only): Same problem - R3 doesn't advertise into Area 0, so this does nothing to protect against R1/R2 receiving the route via R4.
• C (R1 and R2): While this could prevent R1/R2 from using the route, it's the wrong place to filter - the route still propagates through Area 0, and you'd need to repeat the configuration on every new router added. ABR filtering is the architecturally correct solution.

Memory tip: Think "filter at the gate, not at every door." In OSPF, the ABR is the gateway between areas - always filter there to control what enters or exits an area, rather than chasing the problem downstream at each individual router.

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