C
HAPTER
9
| Diagnostics
Traceroute
– 99 –
T
RACEROUTE
Traceroute discovers the routes that packets take when traveling to a
destination. Traceroute works by taking advantage of the error messages
generated by routers when a packet exceeds its time-to-live (TTL) value.
The traceroute command first sends probe datagrams with the TTL value
set at one. This causes the first router to discard the datagram and return
an error message. The trace function then sends several probe messages
at each subsequent TTL level and displays the roundtrip time for each
message. Not all devices respond correctly to probes by returning an “ICMP
port unreachable” message. If the timer goes off before a response is
returned, the trace function prints a series of asterisks and the “Request
Timed Out” message. A long sequence of these messages, terminating only
when the maximum timeout has been reached, may indicate this problem
with the target device. A trace terminates when the destination responds,
when the maximum timeout (TTL) is exceeded, or the maximum number of
hops is exceeded.
Figure 58: Traceroute
The following items are displayed on this page:
◆
Host
— The IP address of the destination host.
◆
Number of Tries
— The number of datagrams to be sent at each TTL
level. The default count is 3. (Range: 1–10)
◆
Timeout
— The number of seconds to wait for a response to a probe
packet. The default is 5000 ms. (Range: 1–65535)
◆
Datasize
— Number of bytes in the packet. The default is 38 bytes.
(Range: 64-1518)
◆
DSCP
— The DSCP value in the IP Header of the packet. (Range: 0-63)
◆
Max Hop Count
— The largest TTL value that can be used. The
traceroute terminates when the destination is reached or when this
value is reached. The default is 30. (Range: 1–255)
◆
Interface
— Selects the interface on which to run the traceroute.
