Which task is performed when troubleshooting LDP?

A primary task when troubleshooting MPLS LDP is to verify the Label Switched Path (LSP) and ensure that labels are being correctly distributed and used for forwarding. The MPLS-aware traceroute utility is a fundamental tool for this purpose. When executed, it provides hop-by-hop information about the path, including the label stack (labels being pushed, swapped, or popped) at each Label Switching Router (LSR). This allows an administrator to confirm that the LSP is built as expected and that traffic is following the correct labeled path, which is essential for diagnosing LDP-related forwarding issues.

A. The ping utility tests Layer 3 reachability and does not provide information about MAC addresses along a path.
B. MPLS must be enabled globally and on interfaces for LDP to function; disabling it would break the network, not troubleshoot it.
D. Cisco Express Forwarding (CEF) is a prerequisite for MPLS on Cisco platforms. A troubleshooting step would be to verify CEF is enabled, not disabled.

1. Cisco, MPLS Label Distribution Protocol (LDP) Configuration Guide, Cisco IOS XE, "Troubleshooting LDP" section. This guide explicitly recommends using the traceroute command to "trace the path that a packet with a specific FEC takes through the network."

2. Cisco, Cisco IOS MPLS Command Reference, traceroute command description. The documentation details how the command can be used in an MPLS environment to display label information for each hop in an LSP.

3. Cisco, Troubleshooting MPLS, Document ID: 25921, "Troubleshooting Tools" section. This document lists ping and traceroute as essential tools, highlighting traceroute for its ability to show the LSP path and labels.