Hi all!

It’s Jared from CCNPJourney.com. Colby had asked me a couple weeks ago if I would be interested in posting some articles on his blog as he’s been fairly busy lately, and of course I said yes. So I thought for my introductory post on the blog I would do a brief write-up on how to use Iperf!

For starters here’s a little bit of info on Iperf. Iperf is a tool that many system/network admins use to measure the bandwidth on a network, as well as the quality of the path on that network. It is capable of generating traffic using TCP and UDP. The TCP and UDP tests are useful for performing the following kinds of tests:

- Latency (response time or RTT): can be measured with the Ping utility.

- Jitter: can be measured with an Iperf UDP test.

- Datagram loss: can again, be measured with an Iperf UDP test.

- Bandwidth tests are done using the Iperf TCP tests.

Iperf also allows you to run simultaneous tests, and bi-directional. Developers in the community have also created a GUI for Iperf called Jperf, which is a Java based GUI that allows you to save settings, and more easily make changes to your settings. For information on Iperf head on over to the Wiki page, or their page over at SourceForge.

Now lets get down to business…

Lets start out by downloading Iperf. In this example I will be using the Windows version of Iperf, but feel free to use what you choose, it’s all the same switches regardless of the OS). You will need Iperf on a machine you wish to use for the “client” role, and one for the server”.

Once you have put the files on each machine we can begin. Start out by opening a command prompt and then navigating to the folder you have Iperf stored in. You will then enter the commands below, on the server and client respectively to begin your first basic Iperf test!


Enter the command “iperf.exe -s”, without the quotes, to start Iperf in server mode (indicated by “-s”).??

Iperf Server Screenshot

The screenshot above shows what you will be presented with after you’ve started Iperf in server mode. It shows you the port that was automatically chosen (which can be manually changed), as well as the TCP window size, which again is chosen by default based on the OS, but can be changed.


To start Iperf in client mode (using no arguments) enter the command “iperf.exe -c x.x.x.x”, where “x.x.x.x” equals the IP address of your Iperf server in the above step.

Iperf Client Screenshot

In the screenshot above you can see the final results of a basic Iperf test. Again presenting you with the TCP Window size and port that were chosen by default. You will also see the resulting bandwidth calculation as well as the amount of data transferred during the test.

Option Usage

Here’s a look at the output from entering the “iperf.exe -h” command.

Usage: iperf [-s|-c host] [options]
       iperf [-h|--help] [-v|--version]
  -f, --format    [kmKM]   format to report: Kbits, Mbits, KBytes, MBytes
  -i, --interval  #        seconds between periodic bandwidth reports
  -l, --len       #[KM]    length of buffer to read or write (default 8 KB)
  -m, --print_mss          print TCP maximum segment size (MTU - TCP/IP header)
  -o, --output    <filename> output the report or error message to this specified file
  -p, --port      #        server port to listen on/connect to
  -u, --udp                use UDP rather than TCP
  -w, --window    #[KM]    TCP window size (socket buffer size)
  -B, --bind      <host>   bind to <host>, an interface or multicast address
  -C, --compatibility      for use with older versions does not sent extra msgs
  -M, --mss       #        set TCP maximum segment size (MTU - 40 bytes)
  -N, --nodelay            set TCP no delay, disabling Nagle's Algorithm
  -V, --IPv6Version        Set the domain to IPv6
Server specific:
  -s, --server             run in server mode
  -D, --daemon             run the server as a daemon
  -R, --remove             remove service in win32
Client specific:
  -b, --bandwidth #[KM]    for UDP, bandwidth to send at in bits/sec
                           (default 1 Mbit/sec, implies -u)
  -c, --client    <host>   run in client mode, connecting to <host>
  -d, --dualtest           Do a bidirectional test simultaneously
  -n, --num       #[KM]    number of bytes to transmit (instead of -t)
  -r, --tradeoff           Do a bidirectional test individually
  -t, --time      #        time in seconds to transmit for (default 10 secs)
  -F, --fileinput <name>   input the data to be transmitted from a file
  -I, --stdin              input the data to be transmitted from stdin
  -L, --listenport #       port to recieve bidirectional tests back on
  -P, --parallel  #        number of parallel client threads to run
  -T, --ttl       #        time-to-live, for multicast (default 1)
  -h, --help               print this message and quit
  -v, --version            print version information and quit
[KM] Indicates options that support a K or M suffix for kilo- or mega-
The TCP window size option can be set by the environment variable
TCP_WINDOW_SIZE. Most other options can be set by an environment variable
IPERF_<long option name>, such as IPERF_BANDWIDTH.

That about sums up this post on Iperf. It’s really a very basic program that can help a lot in day to day troubleshooting. I plan to make another post on how to use Jperf as well, so keep an eye out for that!

Hope you enjoyed!


Jared Miller has been in IT since he started school in 2007. He is currently an Information Systems Analyst at a local hospital and holds the CCNA certification and is beginning to study for the CCNA Voice and CCNP exams, in that order.

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