[c-nsp] MTU at Gb/sec transit and higher

[Gert Doering]

Hi,

On Wed, Mar 17, 2010 at 11:39:01PM -0500, Lawrence E. Bakst wrote:
> I apologize if this is somewhat off-topic. I searched for this and found almost no information. I found this, but it's almost 10 years old and it seems like he gave up:
> http://staff.psc.edu/mathis/MTU/
> 
> 1. If I purchase GbE transit from a provider what size is the MTU likely to be? Is it still ~1500 bytes?

1500.

> 2. At any point closer to the core of the "internet" does the
> MTU step up from 1500 bytes to some larger value and are jumbo
> frames utilized? Or worse does the  MTU step down from 1500 bytes?

Inside of many ISP networks, a bigger MTU is used.  Unfortunately, there
is no "large MTU core" in the Internet yet - many exchange points have
standard 1500 MTU, and all our upstream providers seem to do 1500 as
well (at least, I have not seen anything that indicates otherwise).

> 3. What size MTU is used at (major) peering points?
> 
> I am expecting the answers to all the above to be "it's all ~1500 bytes" 
> but I want to confirm that.

As far as I am aware, the "standard LAN" MTU is 1500.  Some IXPs have
"large MTU" VLANs, but many don't.

> 4. Does most of the current Cisco gear used by NSPs support jumbo frame 
> on GbE and faster interfaces?

For Cisco, the picture is not clear.  The 6500/7600 series does "full jumbo"
with 9000+ bytes.  The 3750 and such seem to do "small jumbo" with less
than 9000 for the routing side of things, and full 9000 for the switching
side (but verify that for yourself).

For the CRS, 12k, and ASR series - no idea.

gert
-- 
USENET is *not* the non-clickable part of WWW!
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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