[vortex] 802.1q VLAN problem
Wed Apr 17 13:28:00 2002
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
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
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.
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