[vortex] Problems with Redhat 7.1 and 3c095c

Donald Becker becker@scyld.com
Mon, 15 Oct 2001 11:29:37 -0400 (EDT)

On Sun, 14 Oct 2001 ruben@nutz.nl wrote:
> On Sun, Oct 14, 2001 at 02:21:07AM -0700, Bill MacAllister wrote:
> > In some notes that I read about setting up ADSL a problem with mentioned
> > with an MTU setting of 1500 and passing packets over PPP.  Basically, all
> > packets were getting fragmented into a big and small packet.  The solution
> > was to drop the MTU.  I tried it and that has fixed the problem that I was
> > seeing.  So, in the end it was the network.
> Which is why I kept insisting on knowing details about your ipchains/tables
> config. If you block ICMP_FRAG_NEEDED-packets you'll effectively disable the
> mechanism IP uses to find the lowest MTU on a link, the MTU-path-discovery. 

Yes, that's very likely what was happening.

> will run into problems. 'Normal' ethernet MTU is 1500 bytes, but after
> encapsulating in PPPoE you have only 1454 bytes left.

Note: PPPoE only takes about 8 bytes, not 46.  So a static mtu=1492 will
usually avoid the problem.

Donald Becker				becker@scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
Annapolis MD 21403			410-990-9993