Time Domain Reflector Guidelines - Cisco Catalyst 9400 System Management Configuration Manual

Troubleshooting the Software Configuration
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.
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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
• Shorted twisted-pair wires—The wires are touching each other or the wires from the remote device. For
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
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.
Example: Performing a Traceroute to an IP Host, on page 417
device.
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.
operating properly
System Management Configuration Guide, Cisco IOS XE Bengaluru 17.4.x (Catalyst 9400 Switches)
Time Domain Reflector Guidelines
to see an example of IP traceroute process.
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