300-510 Question #300: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. An enterprise is running OSPF within its network. Cisco Express Forwarding is enabled on all connected links between routers. Users on the network report that they cannot reach application servers connected to R1. While troubleshooting the problem, a network engineer checked the MPLS forwarding table and noticed that labels are missing for the link between R1 and R4. Which action must the engineer take to resolve the issue?

Options

• AConfigure the OSPF router ID on R1 and R4 as a loopback interface with a /32 mask.
• BPlace the R1-R2 and R1-R4 links into area 0.
• CImplement a virtual link from R1 to either R2 or R4 so that R1 can connect to area 0.
• DEnable LDP on the link between R1 and R4.

Explanation

MPLS label forwarding depends on a label distribution protocol such as LDP. If the R1–R4 interface isn’t running LDP (or you haven’t enabled mpls ip on that link), those two routers won’t exchange labels, so you’ll see the prefix in the RIB but no label in the LFIB. Enabling LDP on that interface (for example via mpls ip under the interface or mpls label protocol ldp) establishes the LDP session and populates the MPLS forwarding table end-to-end.

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