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The traceroute tool provides a UDP-based system for tracing the flow of traffic through a network. traceroute uses the TTL field of the IP header to force each hop along the path to return an ICMP Time Exceeded message. The destination host is recognized because it returns an ICMP Destination Unreachable message.

The first set of packets is sent with a TTL of 1, which times out at the first router. The second set of packets has a TTL of 2 and times out at the second router. This pattern is followed until the destination host is reached.

traceroute at Work

As each packet is sent, the results are displayed.The results of a traceroute session are shown in Listing 5.

Listing 5 traceroute Results

 [pate@router pate]$ traceroute

traceroute to bigfun.whirlycott.com (, 30 hops max,
 38 byte packets
 1 mmgw32.bos1.Level3.net ( 113.711 ms 118.560 ms
 109.549 ms
 2 mmcu32.bos1.Level3.net ( 109.146 ms 109.135 ms
 109.534 ms
 3 gis-gate.gis.net ( 109.215 ms 109.112 ms 109.429 ms
 4 serial2-0-1.hsa1.bos1.Level3.net ( 109.280 ms
 109.070 ms .109.377 ms
 5 lo0.mp2.NewYork1.level3.net ( 119.213 ms 118.905 ms
 109.563 .ms
 6 ( 119.125 ms 118.996 ms 119.365 ms
 7 ATM1-0.BR2.NYC9.ALTER.NET ( 119.551 ms 119.005 ms
 119.413 ms
 8 518.at-5-0-0.XR2.NYC9.ALTER.NET ( 119.254 ms
 108.976 ms .119.354 ms
 9 180.ATM6-0.XR2.BOS1.ALTER.NET ( 119.323 ms 119.070 ms
 119.328 .ms
10 190.ATM9-0-0.GW1.BOS1.ALTER.NET ( 129.204 ms
 118.981 ms .119.454 ms
11 mdc-gw.customer.ALTER.NET ( 149.267 ms 129.104 ms
 129.498 ms
12 bigfun.whirlycott.com ( 139.251 ms 129.062 ms
 139.349 ms

[pate@router pate]$

The first line of output gives a summary of the data to be sent.The following lines show which hop of the path, the hostname and/or IP address of the router for that hop, and the round-trip time for that packet. In this example everything looks pretty good—there are no big jumps in the round-trip time, no packets were dropped, and even the final round-trip time looks pretty small.

The results in Listing 6 aren't quite so healthy, although they still aren't too bad. There are several spikes in the round-trip time and even a dropped packet at the destination host (the * marker).

Listing 6 A Less Healthy Route

[root@cherry /root]# traceroute http://www.vii.com
traceroute to lonepeak.vii.com (, 30 hops max,
 38 byte packets
 1 mango ( 0.504 ms 0.312 ms 0.290 ms
 2 mmgw32.bos1.Level3.net ( 120.978 ms 108.857 ms
 109.181 ms
 3 mmcu32.bos1.Level3.net ( 105.202 ms 112.733 ms
 115.206 ms
 4 gis-gate.gis.net ( 105.302 ms 108.981 ms 1029.911 ms
 5 serial2-0-1.hsa1.bos1.Level3.net ( 114.344 ms 108.968
 ms .118.993 ms
 6 lo0.mp1.Chicago1.level3.net ( 141.197 ms 139.011 ms
 139.223 .ms
 7 ( 141.246 ms 149.155 ms 139.272 ms
 8 aads01.chcg.eli.net ( 214.926 ms 217.502 ms
 199.391 ms
 9 srp2-0.cr01.chcg.eli.net ( 214.816 ms 208.874 ms
 159.308 ms10 .p10-0.cr02.slkc.eli.net (
 214.941 ms 1079.453 ms 214.005 ms
11 gw-VII3-DOM.slkc.eli.net ( 214.463 ms 208.883 ms
 281.402 ms
12 gw-VII3-DOM.slkc.eli.net ( 158.323 ms 208.855 ms
 229.192 ms
13 lonepeak.vii.com ( 1092.011 ms 218.827 ms *
[root@cherry /root]#

Just as many packet filters are configured to block ping, many are configured to stop traceroute from working as well. Although this limits the usefulness of traceroute for end-to-end troubleshooting, traceroute can still provide useful information about the path followed between the endpoints of the connections.

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