Cisco Catalyst 2960 Series, Configuration Manual

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up in the traceroute output if they are only bridging the packet from one port to another within the same VLAN.
However, if the intermediate Switch is a multilayer Switch that is routing a particular packet, this Switch
shows up as a hop in the traceroute output.
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.
Related Topics
Executing IP Traceroute, on page 1644
Example: Performing a Traceroute to an IP Host, on page 1653
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 is supported only on 10/100/1000 copper Ethernet ports. It is not supported on 10-Gigabit Ethernet ports
and on SFP module ports.
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 Switch
• 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 Switch reports accurate information in these situations:
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About Troubleshooting the Software Configuration