Which command is appropriate for troubleshooting a Layer 3 network routing issue?

Using the traceroute command is a standard approach to troubleshoot Layer 3 network routing issues. This command helps you determine the path packets take to reach a specific destination and identifies any points of delay or failure along the route. By providing the number of hops and the response times for each hop, traceroute gives insight into where potential issues may arise in the network, such as unreachable routers or excessive latency.

In comparison, the other commands serve different purposes. Show LLDP neighbor-info is used to display information about directly connected devices that support the Link Layer Discovery Protocol (LLDP), which mainly pertains to Layer 2 connectivity rather than Layer 3 routing. The command snow startup-config appears to be a typographical error and does not correspond to a recognized troubleshooting command; typically, it should be related to displaying or viewing configurations but isn't relevant to routing issues. Config-router is a command used in configuration mode to enter routing protocol settings, but it isn't a diagnostic command and thus would not directly aid in troubleshooting existing routing problems.

By using traceroute, network administrators can effectively diagnose and troubleshoot Layer 3 routing issues to ensure network reliability and performance.