Isolating a routing-path failure You suspect an intermediate router is dropping traffic to a remote network. Which user EXEC command helps you identify the hop at which packets stop being forwarded?

Introduction / Context:
When a destination is unreachable, you must determine where along the path packets are being dropped. Tools that probe each hop and report incremental results are essential to isolate the failing segment. On Cisco devices, traceroute (often abbreviated as trace in user EXEC) performs this function by using increasing TTL values and listening for time-exceeded messages from intermediate routers.

Given Data / Assumptions:

• You have user EXEC access (prompt shows Router>).
• You want to identify the hop count and the node at which forwarding fails.
• ICMP is not being filtered along the path (or UDP probe responses are permitted).

Concept / Approach:

traceroute sends probe packets with incrementing TTL starting at 1. Each router that decrements the TTL to zero returns an ICMP time-exceeded message, revealing its identity. When a hop fails to respond or a later hop is reachable, you can infer where the issue lies. While ping only tells you if an endpoint responds, traceroute shows the entire path, which is far more diagnostic when dealing with multi-hop networks.

Step-by-Step Solution:

Verification / Alternative check:

Use show ip route on the suspect router to verify that a route exists. Confirm with ping sourced from intermediate routers to isolate asymmetric issues.

Why Other Options Are Wrong:

• ping tests reachability but not path details.
• show ip route and show interface provide static information and do not reveal the failing hop across the path.
• pathcheck is not a standard IOS command.

Common Pitfalls:

• Interpreting asterisks as absolute failure; some networks rate-limit or block TTL-exceeded responses.
• Forgetting that traceroute behavior can vary by platform (UDP vs ICMP probes).

Final Answer:

Router> trace