350-401 Question 499
Single answerA network engineer is configuring MPLS in a service provider network. The network consists of Label Switch Routers (LSRs) and uses Label Distribution Protocol (LDP) to establish Label Switched Paths (LSPs). During the configuration, the engineer notices that labeled packets are not being forwarded correctly. Which component is most likely misconfigured, causing the issue?
- A
The LDP session between adjacent LSRs
- B
The routing protocol used to exchange IP prefixes
- C
The label switching mechanism on the ingress LSR
- D
The MPLS forwarding table on the egress LSR
Show answer and explanation
Correct answer: A
Explanation
In MPLS, the LDP session between adjacent LSRs is critical for exchanging label bindings. If this session is misconfigured, the routers cannot establish a label mapping, and as a result, labeled packets cannot traverse the MPLS network. Ensuring proper LDP operation is fundamental to MPLS functionality.
- A. Correct.
If the LDP session between adjacent LSRs is not established, the routers cannot exchange label mappings, which prevents the labeled packets from being forwarded correctly.
- B. Incorrect.
While routing protocols provide the IP prefixes that LDP uses to generate labels, their misconfiguration does not directly impact the forwarding of labeled packets unless the prefixes are missing altogether.
- C. Incorrect.
The ingress LSR assigns labels and initiates the LSP, but incorrect label switching at this point would only affect traffic entering the MPLS network, not the entire forwarding process.
- D. Incorrect.
The egress LSR primarily pops the label and forwards the packet based on IP routing. A misconfiguration here would affect only the final stage of delivery, not the overall LSP.