traceroute [ip-address]
Reveals the exact hop-by-hop path a packet takes through a network to reach its destination. If a ping fails, traceroute tells you exactly which router in the middle of the chain dropped the packet.
How Traceroute Exploits TTL
Traceroute is a clever hack using the IPv4 Time To Live (TTL) field. The router intentionally sends three packets with a TTL of 1.
The very first router in the path receives the packet, decrements the TTL to 0, drops the packet, and sends back an ICMP "Time Exceeded" error. Traceroute records the IP address of that error message as Hop 1.
It then sends three packets with a TTL of 2. The first router decrements to 1 and forwards it. The second router decrements to 0, drops it, and replies. This process repeats, incrementally increasing the TTL until the destination is finally reached.
CCNA Exam Gotchas
Cisco UDP vs Windows ICMP
This is a classic trivia question: While the Windows Command Prompt utility (tracert) uses ICMP Echo Requests to probe the path, Cisco IOS traceroute generates UDP packets aimed at a high, invalid destination port (33434+). Security firewalls often block UDP but allow ICMP, meaning a Windows tracert might succeed while a Cisco traceroute fails.