D-Link DGS-1520 Emulator - Page 523

DGS-1520 Series Gigabit Ethernet Smart Managed Switch CLI Reference Guide Command Default Level Level: 1. Usage Guideline To terminate the traceroute before it has finished, press CTRL+C. Up to three active traceroute sessions are supported. This command uses the TTL field in the IP header to cause routers and servers to generate specific return messages. A traceroute starts by sending a UDP datagram to the destination host with the TTL field set to 1. If a router finds a TTL value of 1 or 0, it drops the datagram and sends back an ICMP time-exceeded message to the sender. The traceroute facility determines the address of the first hop by examining the source address field of the ICMP time-exceeded message. To identify the next hop, traceroute again sends a UDP packet, but this time with a TTL value of 2. The first router decrements the TTL field by 1 and send the datagram to the next router. The second router sees a TTL value of 1, discards the datagram, and returns the time-exceeded message to the source. This process continues until the TTL is incremented to a value large enough for the datagram to reach the destination host (or until the maximum TTL is reached). To determine when a datagram has reached its destination, traceroute sets the UDP destination port in the datagram to a very large value that the destination host is unlikely to be using. When a host receives a datagram with an unrecognized port number, it sends an ICMP port unreachable error to the source. This message indicates to the traceroute facility that it has reached the destination. Example This example shows how to trace-route the host 172.50.71.123. Switch#traceroute 172.50.71.123