jumbo frames

Dave Siegel dave at siegelie.com
Wed May 30 21:32:38 UTC 2001

> > I've seen a lot of discussion about why one would want to do Jumbo
> > frames on your backbone...let's assume for the sake of argument that a
> > customer requirement is to support 9000 bytes packets across your
> > backbone, without fragmentation.
> > 
> > Why not bump MTU up to 9000 on your backbone interfaces (assuming they
> > support it)?
> > 
> > What negative affects might this have on your network?
> > 
> > a) performance delivering average packet sizes
> > b) QoS
> > c) buffer/pkt memory utilization
> > d) other areas
> Theoretically increasing the MTU anywhere you are not actually generating
> packets should have no impact except to prevent unnecessary fragmentation.

Well, yes.

But let's consider then the impact of forwarding packets into the same pipe
where the sizes are more than two orders of magnitude apart? (60 vs. 9000).

> But then again, theoretically IOS shouldn't get buggier with each release.

Let's not focus on the OS being used, and assume it's a well designed OS.
For the sake of argument, let's say it's a simulated OS.

> There will obviously be different packet handling techniques for the
> larger packets, 

Why will there "obviously" be different packet handling techniques?
is there a special routine for packets over 1500 bytes?  4470 bytes?
Or do you mean policy mapping jumbo packets into dedicated queues?

> and I'm not aware of any performance or stability testing
> that has been done for jumbo frames. I'm guessing the people who are
> actively using them havn't been putting it in line rate mixed small and
> large packets conditions.

Probably not, but if you take a person who is using them and let them use
your backbone as a point to point network, you start to see that...not at
line rate (preferably, anyway) but certainly large quantities of small
packets mixed in with some really big ones.

Where your possible QoS impact comes in is where you have jumbo packets
sitting in the best effort queue, and small frames sitting in your priority
queue, you have some queue contention.  This doesn't happen so much as a 
result of having the large packet there, but rather because your minimum queue
size has to be of sufficient size to carry the packet.  The linecard is
going to spend more time draining the best effort queue while the priority
queue gets stacked.

Even without QoS, what do jumbo frames do to overall buffer utilization?
(I would imagine nothing, since I suspect this is a function of line speed,
not packet size).


Dave Siegel
HOME   520-579-0450   dave at siegelie dot com
WORK   520-572-9041   dsiegel at gblx dot net
                      Director, Global IP Network Design, Global Crossing

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