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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 37 Troubleshooting
Diagnosing Connectivity Problems
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 appears 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.
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.
Command
Purpose
traceroute ip host
Trace the path packets take through the network by using IP.