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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 27 Troubleshooting
Diagnosing Connectivity Problems
This example shows how to ping an IP host:
Switch# ping 172.20.52.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echoes to 172.20.52.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Switch#
Table 27-1
describes the possible ping character output.
To terminate a ping session, enter the escape sequence (Ctrl-^ X by default). You enter the default by
simultaneously pressing and releasing the Ctrl, Shift, and 6 keys, and then pressing the X key.
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).
Table 27-1
Ping Output Display Characters
Character
Description
!
Each exclamation point means receipt of a reply.
.
Each period means the network server timed out while waiting for a reply.
U
A destination unreachable error PDU was received.
C
A congestion experienced packet was received.
I
User interrupted test.
?
Unknown packet type.
&
Packet lifetime exceeded.
