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Archive for the ‘Troubleshooting’ Category

Layer 1 Troubleshooting with Cisco Switches.

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Apparently Cisco switches have the capability of performing TDR tests on cables to test cable length and pin-outs. This is something I stumbled upon while browsing through the Cisco Learning Network and I just had to give it a whirl. After all sooner or later you are going to run into a cable issue, it’s just a matter of time. Whether it was not crimped, punched down correctly, or someone ignored the IEEE standard and made a cable too long. The bottom line is some user somewhere will not connect to the network and then it will be up to you to troubleshoot why (and to fix it).

To test a cable simply issue the following command: test cable-diagnostics tdr int %IntefaceType/Number% from privileged mode, and include the interface in question.

After a few seconds issue the following command to see the results: Sh cable-diagnostics tdr int %IntefaceType/Number%

The results are as follows:

Pretty straight forward, it tells you the interface the cable is plugged into, the speed of the connection, the local and remote pair of the cables, cable length (notice the “+/-” of a few meters), and finally the status.

Here is a description of the results from Cisco’s website (Found here):

This command is currently available many different switch models 2900’s,  3500’s, 3700’s, and further (Including both 10/100 MB and 1 GB ports). I would also like to mention if you perform this test on a live port, communication is not interrupted at all. The Cable Length measurement is actually very accurate, don’t let the “+/-” factor confuse you, I’ve ran this on multiple cables and the length calculation is right on the mark. This is a definitely a nifty feature to keep in your when you run into those wired connectivity issues.

Written by Stephen J. Occhiogrosso

April 18, 2011 at 3:17 PM

Who’s congesting my network?

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I figured I would write a post concerning some features built-in to most Cisco routers nowadays that can be lifesavers in identifying network congestion and who/what is causing it.

The first feature I want to mention is NetFlow, this nifty little feature will identify network traffic by the protocol as well as determine how much throughput each protocol is using giving you a clear view of the traffic traveling your network. You configre it on a per interface basis, specify the address you want the Netflow information sent to, and also the port you want it sent out on. 2055 is the default port used by the SolarWinds Netflow Analyzer in this case (Free Tool)

You can issue the sh ip cache flow command to see the output. While this output can be duanting at first it is actually fairly simply to understand once you realize what each column signifies. A nice shortcut for analyzing netflow is to find a free tool that will do it for you.

Their is more information displayed but from this point it looks almost identical to the sh ip flow top-talkers command shown below, the important thing here is the breakdown of the major protocols.

The next really cool feature is called top talkers after you configure this you can quickly see which end devces on your network are taking up the most bandwidth.

The configuration is as follows:

A fairly straight forward configuration, first you enable top top talkers and then configure the parameters you want. You can set top-talkers to sort by the amount of bytes from each end device or by the amount of packets. You can also configure the amount of devices you want to see, anything from 1 device to 200 device I usually prefer to simply see the top 10 devices (well 8 in this case)

You view the top talkers with the sh ip flow top-talkers command:

As you can see the output is placed nicely in a few columns, identifying the source interface and IP address, the destination interface and IP address,  the protocol number (Pr column), the source and destination ports (keep in mind these are in hex format and need to be converted to decimal), and lastly the amount of bytes transferred in this case.

So whether someone has introduced a new program, or a users decides to try and download the entire internet you should be able to easily identify it. Those two built-in features alone can help you troubleshoot any network congestion your network experiences with your Cisco devices.

Written by Stephen J. Occhiogrosso

January 13, 2011 at 1:13 PM

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