Subject: Re: 802.1Q & ETHER_MAX_LEN
To: Jason R Thorpe <>
From: Manuel Bouyer <>
List: tech-net
Date: 10/03/2000 17:29:34
On Tue, Oct 03, 2000 at 07:17:33AM -0700, Jason R Thorpe wrote:
> On Tue, Oct 03, 2000 at 03:12:10PM +0200, Manuel Bouyer wrote:
>  > Is there other case where we need something like this (different MTU at
>  > the hardware level depending on the frame type) ? If not maybe we could just
>  > hack something simpler: change ifnet (or maybe, really if_data) to include
>  > the max frame length the HW can do; which may be bigger than
>  > if_mtu + if_hdrlen. Then extra encap layers such as 802.1Q can use it to
>  > recompute a MTU. Anyway we won't be able to have a completely clean separation
>  > between 802.1Q and ethernet.
> How is this problem really any different than e.g. FDDI using a different
> MTU for IP than it does for anything else?

How is this handled ?

The difference I can see if that, with fddi, we know the max len; for IP and
for anything else. In our case, the driver needs to inform if_vlan whenever
it can do 1522 or only 1518. Not all ethernet boards can do 1522.

Manuel Bouyer, LIP6, Universite Paris VI.