Displaying The Physical Path; Using Ip Traceroute; Understanding Ip Traceroute - Cisco WS-C3560-48PS-S Software Configuration Manual

Using IP Traceroute

Displaying the Physical Path

You can display physical path that a packet takes from a source device to a destination device by using
one of these privileged EXEC commands:
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For more information, refer to the command reference for this release.
Using IP Traceroute
This section consists of this information:
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Understanding IP Traceroute

You can use IP traceroute to identify the path that packets take through the network on a hop-by-hop
basis. The command output displays all network layer (Layer 3) devices, such as routers, that the traffic
passes through on the way to the destination.
Your switches can participate as the source or destination of the traceroute privileged EXEC command
and might or might not appear as a hop in the traceroute command output. If the switch is the destination
of the traceroute, it is displayed as the final destination in the traceroute output. Intermediate switches
do not show 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 back an Internet Control Message Protocol (ICMP)
time-to-live-exceeded message to the sender. Traceroute determines 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 determine 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 this message was sent by the
destination.
Catalyst 3560 Switch Software Configuration Guide
35-16
tracetroute mac [interface interface-id] {source-mac-address} [interface interface-id]
{destination-mac-address} [vlan vlan-id] [detail]
tracetroute mac ip {source-ip-address | source-hostname}{destination-ip-address |
destination-hostname} [detail]
Understanding IP Traceroute, page 35-16
Executing IP Traceroute, page 35-17
Chapter 35 Troubleshooting
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