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Troubleshooting Commands
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The TTL is then incremented to two and sent out. The packet transverses
the first hop, the TTL is decremented to one, and it is forwarded on to the
next-hop. When the second hop receives the packet, the TTL is decremented
to zero and the error message is sent to the source address.
This process is followed until the destination host responds or until the TTL
is exceeded. By default, the maximum TTL is 30. This means that if the destina-
tion host does not respond, the traceroute utility will attempt 30 times. Multiple
requests are sent each attempt, which results in three RTT responses. In addition
to the TTL error messages, port unreachable messages provide sufficient infor-
mation for a path to the destination.
Table 4.17 lists the explanation for the response characters available
within the traceroute utility.
Successful functionality of the traceroute command depends on the IP
configuration on each host along the path to the destination. It is possible
that the IP configuration does not send error messages when the TTL
expires, TTL is not decremented, or no port unreachable messages are sent.
If any of these problems exist, you probably get timeout responses.
T A B L E 4 . 1 7
traceroute Response Meanings
Character
Explanation
xx msec
The RTT for each packet
*
Timeout
H
Host Unreachable
U
Port Unreachable
N
Network Unreachable
P
Protocol Unreachable
A
Administratively denied
Q
Source Quench
?
Unknown packet type
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