Subject: Re: 802.1Q & ETHER_MAX_LEN
To: Manuel Bouyer <>
From: Jasper Wallace <>
List: tech-net
Date: 10/02/2000 13:18:56
On Sun, 1 Oct 2000, Manuel Bouyer wrote:

> Hi,
> I tested our 802.1Q support with a cisco switch today, and ran into the
> problem mentionned in kern/11109 (I expected to run into this anyway :),
> which is that on a cisco trunk port we can expect to see packets of
> 1522 bytes instead of 1518 (the 4 extra bytes being the 802.1Q tag).
> If we want to be able to talk with others 802.1Q devices we need to solve
> this.
> 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 ?

fxp's can, the older ones can be told to ignore large frames, and newer ones
can fo hardware vlan tagging/detagging.

It's the same for recent 3com cards.

Patches for both (for freebsd) are here:

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