[c-nsp] multiple vlans on a port

[Gert Doering]

Hi,

On Mon, Jul 13, 2009 at 06:38:23PM -0400, Matthew Huff wrote:
> Also, with 802.1q framing, you might run into fragmentation on
> the non-native VLANs. You may want to adjust the MTU on the virtual
> machines if Linux doesn't do it automatically.

There are a few broken NIC cards on the Linux side that have issues
with "baby-jumbo" packets (1500 + 4 byte for 802.1q header).  Decent
gear - and that's what you want to use on a *server* - doesn't have
any issues there.

And, just to clarify: *If* you have MTU problems due to 802.1q headers,
you will not see "fragmentation".  You'll see black-holing, because the
stack will not know about the MTU issue, and thus won't even think
about fragmentation.  (Fragmentation happens if there is a link on
the path that has smaller L3 MTU than the packet's sender - but in this
scenario, the L3 endpoints assume 1500, while the L2 link cannot handle
this.  Black hole).

gert
-- 
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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