350-501 Question #404: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. Router 1 and Router 2 were installed in the data center Router 1 is the core router in the network, but it fails to establish an OSPF peering with Router 2, and customer traffic is unable to pass. Router 1 also reports an increase in CPU and memory usage. However, the CPU for R2 is stable. Which action resolves this issue?

Options

• AChange the MTU to 1524 on Router 1.
• BDisable Cisco Express Forwarding on Router 2.
• CChange the transport mode to WAN on Rooter 1.
• DEnable MPLS on Router 2.

Explanation

The OSPF peering failure between R1 and R2, coupled with high CPU and memory usage on R1 while R2 remains stable, strongly indicates an MTU mismatch. R1 likely has a smaller MTU and is struggling to process larger OSPF Database Description (DBD) packets sent by R2. Aligning R1's MTU to match R2's (e.g., 1524) will resolve the peering issue.

Common mistakes.

• B. Disabling Cisco Express Forwarding (CEF) would typically increase CPU usage for packet forwarding rather than resolve an OSPF peering issue or CPU spikes on a different router.
• C. "Transport mode" is not a standard OSPF configuration setting that would directly affect peering establishment.
• D. Enabling MPLS on Router 2 is related to label switching and has no direct bearing on resolving OSPF peering failures or high CPU utilization caused by MTU mismatches.

Concept tested. OSPF MTU mismatch troubleshooting

Reference. https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13689-19.html

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