Testing LFNs

I have a new T3 thats 65msec long. I'd usually be using iperf to test new
links, but at 65msec, even at the maximum window size, I can only get
6-8mbit through. No combination of options I've been able to find has gotten
me more than 6mbit through this link. Should I just shotgun 9 copies of it?

Are there better ways to test these links? Can one verify this link with
just a pair of 7200s and linux machines on either side? Or is this something
one really needs "real" test hardware for? If 6mbit go through clean, is
there a real chance 44 will not?


set your system send and receive TCP buffers larger. You're probably being limited by that. With linux, make sure you have window auto-scaling enabled and have increased the maximum size it can grow to to at least 4MB.

Or test with UDP and blast as fast as you can so that you're not seeing TCP weirdness.

IOS includes a 'ttcp' command you can use on the router itself:



Thanks to everyone who responded on and off-list!
It seems evident that I didn't have a complete understanding of the iperf
switches which alter buffer sizes.

Several people made a few neat points, which I'll quickly summarize:

* In iperf, -P will allow one to run multiple tcp tests at once.
* IOS has a built-in tester... ttcp. http://tinyurl.com/6fp75j
* For suggestions on changing the kernel buffer sizes:
* . http://www.29west.com/docs/THPM/ (section 8)
* . http://fasterdata.es.net/
* Linux.com has a related writeup: http://www.linux.com/feature/144532

Finally, "past performance does not indicate future results" applies here.
44mbit will not necessarily go clean just because 6 did.

Thanks for the tips,

That's the best option... spew packets. Just make sure they are as large as possible without needing fragmentation.

And if your setup can support it, set the ethernet MTUs to match the serial interface (4470). Not that PPS should be an issue -- even an ancient NPE150 should be able to process switch enough packets to flood a T3.