Head in a blender

Wrong MTU makes Notes look slow, but it’s not true

Andy Pedisich  November 3 2008 02:40:51 PM
This is just a short network geek post.

Once day, a few weeks ago, a couple of systems started to slow down when accessing the Internet.   It was most painfully evident when sending a large attachment using Notes.  It would have been with any mail, but of course for me it was Notes.

I did a little testing using DSL Reports and Speedtest.net and found that while my download speed seemed OK, the upload speed was a deplorable 128 KBPS.  I immediately called ComicalCast, thinking that they were screwing around with upload speeds.

ComicalCast Support checked the cable modem and said everything was peachy, which of course didn't assuage me in the least.

I followed a bunch of Google and Yahoo threads, all of which were dead-ends, except there was a mention of adjusting the MTU, or Maximum Transmission Unit.  I had adjusted the MTU of a couple of routers about 12 years ago when I worked for someone else.  Back then, I couldn't get Domino servers to stay connected between sites without lowering the default MTU setting.

Since the problem was happening on more than one system, I decided to look at the router/firewall first.  Mind you, this hadn't been touched, nor had any of the desktops and laptops that were exhibiting this slow upload behavior.  A 10 MB file was taking over a half hour to get up there!

The default MTU was set to 1500.  I lowered it down to 900. Uploads were faster.  Then I kept adjusting it up until the speed fell off again.  I settled on 1200.  That's only a mere 300 bytes lower than the default, but now the 10 MB uploads in less than 30 seconds.  Upload speed went from 128 KBPS to close to 3,000 KBPS.

The only reason I mention it is because I can never tell if the MS Updates are messing with me or not.  After all, the updates do re-set my default mail to some other product every time one comes by.

Check your upload and download, and make sure you're getting what you're supposed to.  I get tired of hearing, "My Notes is so slow," when it's something else at fault.

- Andy

PS: Don't forget to VOTE!


1Gregg Eldred  11/3/2008 4:04:56 PM  Wrong MTU makes Notes look slow, but it’s not true

You can get this internally as well, with mis-configured routers. I have used this TechNote to "prove" that Notes isn't the issue, just the reporting application.

{ http://www-01.ibm.com/support/docview.wss?uid=swg21086718 }

But then you need to get your internal networking people to acknowledge the issue and fix it. :-)

2Andy Pedisich  11/3/2008 4:51:55 PM  Wrong MTU makes Notes look slow, but it’s not true

Thanks very much for including the link, Greg. I bet many Notes adminministrators are trying it right now.

- Andy