The traceroute privileged EXEC command uses the Time To Live (TTL) field in the IP header to cause routers and servers to generate specific return messages. Traceroute starts by sending a User Datagram Protocol (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 an Internet Control Message Protocol (ICMP) time-to-live-exceeded message to the sender. Traceroute finds the address of the first hop by examining the source address field of the ICMP time-to-live-exceeded message. To identify the next hop, traceroute sends a UDP packet with a TTL value of 2.
The first router decrements the TTL field by 1 and sends the datagram to the next router. The second router sees a TTL value of 1, discards the datagram, and returns the time-to-live-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 learn when a datagram reaches its destination, traceroute sets the UDP destination port number in the datagram to a very large value that the destination host is unlikely to be using.
When a host receives a datagram destined to itself containing a destination port number that is unused locally, it sends an ICMP port-unreachable error to the source. Because all errors except port-unreachable errors come from intermediate hops, the receipt of a port-unreachable error means that this message was sent by the destination port. Go to Example: Performing a Traceroute to an IP Host, on page 28 to see an example of IP traceroute process. Time Domain Reflector Guidelines You can use the Time Domain Reflector (TDR) feature to diagnose and resolve cabling problems. When running TDR, a local device sends a signal through a cable and compares the reflected signal to the initial signal.
TDR can detect these cabling problems: • Open, broken, or cut twisted-pair wires—The wires are not connected to the wires from the remote device. • Shorted twisted-pair wires—The wires are touching each other or the wires from the remote device. For example, a shorted twisted pair can occur if one wire of the twisted pair is soldered to the other wire. If one of the twisted-pair wires is open, TDR can find the length at which the wire is open. Use TDR to diagnose and resolve cabling problems in these situations: • Replacing a device.
• Setting up a wiring closet • Troubleshooting a connection between two devices when a link cannot be established or when it is not operating properly When you run TDR, the device reports accurate information in these situations: • The cable for the gigabit link is a solid-core cable. • The open-ended cable is not terminated. When you run TDR, the device does not report accurate information in these situations: • The cable for the gigabit link is a twisted-pair cable or is in series with a solid-core cable. • The link is a 10-megabit or a 100-megabit link. Troubleshooting the Software Configuration Time Domain Reflector Guidelines Troubleshooting the Software Configuration 5