[vortex] 802.1q VLAN problem

SCHIMMEL Guillermo TECHTEL gschimmel@techtel.com.ar
Wed Apr 17 14:55:01 2002

Hi, and thank you for taking the time to answer my question.

The cards that we are using are of a lot of models (all 3com). However we
can use the 3c905C or whichever that can actually understand the vlan frame.

I do not intend to start  playing with 4k packets, I just want to do inter
vlan routing with a PC and a Cisco 2900 switch.

As in the same PC I also have a STM-1 card, I'll save a lot of money. It's a
shame that the only limitation to that is that the driver doesn't like that
4 bytes. More even knowing that the hardware itself supports that.

Any help would be really apreciated.

Thanks again for your time.


> -----Mensaje original-----
> De: Bogdan Costescu [mailto:bogdan.costescu@iwr.uni-heidelberg.de]
> Enviado el: Wednesday, April 17, 2002 2:27 PM
> Para: SCHIMMEL Guillermo TECHTEL
> Cc: 'vortex@scyld.com'
> Asunto: Re: [vortex] 802.1q VLAN problem
> On Mon, 15 Apr 2002, SCHIMMEL Guillermo     TECHTEL wrote:
> > 	Recentrly we have started tests in order to develop a vlan based
> > network around the 3com vortex card.
> > 	The problem is that the driver doesn't like the 
> oversized 1504 bytes
> > packets that the 802.1q module gives to him.
> > 
> > 	Are you aware of that problem? Do you intend to patch 
> the driver?
> Yes and maybe. First of all, you didn't say which cards/chips you are 
> using; both Cyclone (905B) and Tornado (905C) have some support for 
> over-sized frames, but I don't know about the older ones (as the 
> expression "3Com vortex card" covers quite a lot of them).
> I don't have the documentation at hand so the following is just from 
> memory:
> Cyclone and Tornado have support for up to FDDI (4.5 Kb) frames; 
> there is one bit which enables this behaviour. However, I 
> don't know if 
> the VLAN support really needs to have over-sized frame 
> support activated. 
> There is support for ignoring the 4 bytes VLAN tag when computing 
> checksums (in the Rx path or zero-copy Tx path) and this can also be 
> enabled easily.
> There were some messages on netdev list some weeks ago (unfortunately 
> the archive at http://oss.sgi.com/projects/netdev/archive/ 
> seems to be 
> dead, maybe you can find another one) related to this which 
> included a 
> link to a patch.
> My feelings on the matter are mixed: I do understand that 
> VLANs are useful 
> (or even required) in some situations or that generally speaking 
> over-sized frames might be useful for more efficient 
> transmission (less 
> header overhead). However, enabling over-sized frames (there 
> is nothing 
> like allow over-size frames up to a variable limit; you can 
> only turn on 
> or off the over-sized frame support) can have bad effects due 
> to: limited 
> FIFO buffers on the card (2 Kib), limited buffers on the 
> switch (today's 
> switches are almost all store-and-forward), incompatibilities 
> which can 
> appear if the recipient(s) of the packet do(es) not have support for 
> over-sized frames (I think that there are switches that do GB 
> Jumbo -> 
> Ethernet conversions, but there are not some to do arbitrary size -> 
> Ethernet conversions). In any case, I'd be willing to spend some time 
> helping adding support to the driver.
> Bogdan Costescu
> IWR - Interdisziplinaeres Zentrum fuer Wissenschaftliches Rechnen
> Universitaet Heidelberg, INF 368, D-69120 Heidelberg, GERMANY
> Telephone: +49 6221 54 8869, Telefax: +49 6221 54 8868
> E-mail: Bogdan.Costescu@IWR.Uni-Heidelberg.De