350-401 Question #276: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. The traceroute fails from R1 to R3. What is the cause of the failure?

Options

• AThe loopback on R3 is in a shutdown stale.
• BAn ACL applied Inbound on loopback0 of R2 Is dropping the traffic.
• CAn ACL applied Inbound on fa0/1 of R3 is dropping the traffic.
• DRedistribution of connected routes into OSPF is not configured.

Explanation

Traceroute Failure Analysis

Option C is correct because the traceroute output shows that packets successfully reach R3's fa0/1 interface (the path is reachable up to that point), but traffic is being dropped by an inbound ACL applied on R3's fa0/1 - this is confirmed when the traceroute reaches R3 but fails to complete, indicating the packet arrives but is filtered before being processed or forwarded to the destination loopback.

Why the distractors are wrong:

• Option A is incorrect because a shutdown loopback would still show the interface in the routing table as down/down; the traceroute behavior shown suggests routing is functioning but traffic is being filtered, not that the destination interface is simply off.
• Option B is incorrect because an inbound ACL on R2's loopback0 would block traffic destined to or sourced from R2's loopback, not affect transit traffic toward R3.
• Option D is incorrect because if redistribution were missing, the loopback route simply wouldn't appear in the routing table, causing an immediate "network unreachable" error rather than the partial traceroute success shown.

💡 Memory Tip: Think "last hop, last filter" - when a traceroute reaches its final router but fails, always suspect an inbound ACL on the receiving interface of the destination router, as it's the last point where traffic can be blocked before reaching its target.

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