Subject: Re: 802.1Q & ETHER_MAX_LEN
To: Matt Thomas <>
From: Manuel Bouyer <>
List: tech-net
Date: 10/02/2000 10:01:42
On Sun, Oct 01, 2000 at 12:48:04PM -0700, Matt Thomas wrote:
> > [...]
> >There are several issue here:
> >- hardware: does the ethernet adapter supports frames of 1522 bytes ?
> >   I looked quickly at the tulip and smc83c170 sources and found no way to
> >   change this, it looks like the 1518 bytes values is hardcoded in hardware.
> >   Does anyone know if there are (10/100) ethernet chips that can send/receive
> >   frames bigger than the standart len ?
> The tulip can.  For receive, you just need to ignore the frame length error.
> the de driver has code to do that.

Ok, I ignored the frame length error and now I can talk 802.1q with the
cisco without troubles (using NFS, etc).

> >- software: assuming there are adapters that can do something else than 1518
> >   (gigabit adapters can), we need a way to advertize this to the vlan layer.
> >   For now the vlan layer uses the MTU advertised by the ethernet driver.
> >   We can't change this (from 1500 to 1504) because non-802.1Q packets would
> >   be too big. So we may need a new field in the if_data struct (maybe 
> > ifi_rmtu)
> >   to advertise the real MTU the interface can do, and which can be used by
> >   pseudo-interfaces like vlan.
> Personally, I'd rather have a IFF_ flag that indicates this instead.  Not 
> all 802
> media is ethernet.

I don't understant this. All interface have an MTU, rigth ?
I can't see what a IFF_ flag would give us. To enable/disable the "long frame"
feature ?
Anyway we have 2 things: the MTU we want to advertize to protocol layers,
and the MTU the interface can really do. We need a way to advertize both and
this, independantly from a IFF_ flag.

Manuel Bouyer, LIP6, Universite Paris VI.