Using the Tracert utility
columns show the time it took for the 070-486 router to respond for each attempt. The last column lists the fully qualified domain name of that router, which typically gives you an indication of where the router is. For example, looking at Figure 6-10 again you can see that hop number 12 looks like it is in Chicago!
Suppose you cannot access a particular web site on the Internet. Your company is directly connected to the Internet via an ISDN line, which is used by approximately 35 people. You are able to hit certain web sites consistently, but others are available only sporadically. Other users begin to notice that they are unable to connect to the same web sites with which you are having a problem. Tracert fits in well here to begin isolating where the problem is. Although you might have a good idea of what network equipment and options are used within your company, once packets enter the Internet, there is no telling what they might come across. Because routes can be so dynamic, this is a great tool for figuring out where the data is traveling to reach its destination.
You can begin troubleshooting this problem by typing one of the following commands:
After the utility has run, you might notice the following entry on one of the routers along the way:
Destination Net Unreachable
The tracert -d form indicates to the utility that you do not want to display hostnames with the tracert output. Specifying -d will speed up the trace because each IP address does not need to be resolved to a name.
Although this utility is unable to determine why the error is occurring, it has effectively found at what point the problem exists. Armed with this information, the owner can then examine that router to resolve the issue. In the event a name resolution method is not available for remote hosts, you can specify the -d option to prohibit the utility from trying to resolve hostnames as it runs. Without this switch, the program will still work, but it will attempt to translate every hop’s IP address to a hostname, thereby slowing the process.
To practice Tracert, check out Exercise 6-4 from the LabBook.pdf on the CD-ROM or watch the CertCam training video on the CD-ROM.
Maximum Number of Hops
One function of the Tracert utility is to provide the number of hops, or networks, that the data is crossing. You may want to limit the number of hops the program will make to search for the remote host; if you don’t, Tracert will continue for 30 hops by default. In the unusual case that you must surpass 30 hops, you can also specify a greater interval. By using the -h option with Tracert, you can specify the maximum number of hops to trace a route to. Figure 6-11 shows an example of limiting the number of hops returned.
Specifying the maximum number of hops with Tracert
Note that some ISPs will not allow an ICMP packet to pass through their gateway, so you may get some lines with * showing there was a hop there but with no information displayed.
Adjusting Timeout Values
Another option associated with Tracert is to adjust the timeout value using the -w switch. This value determines the amount of time in milliseconds the program will wait for a response before moving on. Using this option will enable you to understand a little more about the problem that is occurring. For example, if you notice that many responses are timing out, you can raise this value. If, after raising this value, remote devices are responding, this could be a good indication that you have a bandwidth problem.
Loose Source Routing Options
An additional option is to use what is known as loose source routing. The -j option can be used to force the outbound datagram to pass through a specific router and back. This enables you to trace the round-trip route for a destination. A normal tracert follows the route until it reaches its destination or times out. When you specify this option, Tracert follows the path to the router specified and returns to your computer. To use loose source routing, enter the following command:
TRACERT -j <hop list>
Tracert can be a useful tool in determining why a remote host can’t be reached. It can also be a good tool to notice other issues, such as bandwidth utilization problems. Its additional options and functionality make it a powerful tool. Table 6-4 lists the Tracert switches and the corresponding definitions.
It is important to note that the Tracert command is used in Windows.The equivalent command in the Linux world is traceroute.
-d -h -j -w -target_name
Does not resolve address to computer names. Specifies maximum number of hops. Specifies loose source route along host-list. Specifies time in milliseconds to wait for reply. Specifies target computer.
Tracert Command-Line Switches
CERTIFICATION OBJECTIVE 6.05
Now that you’ve learned how to trace data 070-486 exam dumps packets throughout the network, another utility useful for troubleshooting TCP/IP-specific issues is Netstat. Netstat displays protocol statistics and current TCP/IP network connections. This utility can be used to display in-depth detail about protocol status and statistics for the different network interfaces, as well as viewing the current listening ports and the routing table.