Tuesday, May 03, 2011


iperf is a tool for testing the throughput of a network pipe. It came in handy today as I was testing out the throughput of our WAN lines.  We have T3’s at each site and one site in particular always “seemed” slow when transferring files but I didn’t know why.  I installed iperf on linux (yum install iperf) and set one side as a server.

iperf –s

Then on another box on the other side of the wire I installed iperf and ran it as a client":

iperf –c x.x.x.x –d  (Where x.x.x.x is the ip address of the box running as a server.)

The I got the following result:

C:\>iperf -c x.x.x.x -d
Server listening on TCP port 5001
TCP window size: 8.00 KByte (default)
Client connecting to x.x.x.x, TCP port 5001
TCP window size: 8.00 KByte (default)
[1840] local y.y.y.y port 1552 connected with x.x.x.x port 5001
[1816] local y.y.y.y port 5001 connected with x.x.x.x port 46524
[ ID] Interval       Transfer     Bandwidth
[1816]  0.0-10.0 sec   439 MBytes   8.5 Mbits/sec
[1840]  0.0-10.0 sec   321 MBytes   6.7 Mbits/sec

This is a full duplex T3 line so I expect something higher than 8.5 and 6.7.  After some inspection I noticed that switchport on the backbone switch was set for Auto-10 instead of just Auto.  That was restricting it to Ethernet speeds. I changed it to Auto and it picked right back up (~30Mbits/sec).  I did this while other traffic was on the wire so it couldn’t entirely fill the pipe by itself.

Cool tool!

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