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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 27 Troubleshooting
Diagnosing Connectivity Problems
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.
Executing IP Traceroute
Beginning in privileged EXEC mode, follow this step to trace the path packets take through the network:
Note
Though other protocol keywords are available with the traceroute privileged EXEC command, they
are not supported in this release.
This example shows how to perform a traceroute to an IP host:
Switch# traceroute ip 171.9.15.10
Type escape sequence to abort.
Tracing the route to 171.69.115.10
1 172.2.52.1 0 msec 0 msec 4 msec
2 172.2.1.203 12 msec 8 msec 0 msec
3 171.9.16.6 4 msec 0 msec 0 msec
4 171.9.4.5 0 msec 4 msec 0 msec
5 171.9.121.34 0 msec 4 msec 4 msec
6 171.9.15.9 120 msec 132 msec 128 msec
7 171.9.15.10 132 msec 128 msec 128 msec
Switch#
The display shows the hop count, IP address of the router, and the round-trip time in milliseconds for
each of the three probes that are sent.
Command
Purpose
Step 1
traceroute ip host
Trace the path packets take through the network by using IP.
Table 27-2
Traceroute Output Display Characters
Character
Description
*
The probe timed out.
?
Unknown packet type.
A
Administratively unreachable. Usually, this output means that an access list is
blocking traffic.
H
Host unreachable.
N
Network unreachable.
P
Protocol unreachable.
Q
Source quench.
U
Port unreachable.
