VLANs with Xen - work with 1 Gbit, doesn't with 100 Mbit cards?

[Tomasz Chmielewski <mangoo@wpkg.org>]

I have a strange problem with VLANs with Xen 3.0.4. I signalled it on
Xen-users list, but found no definitive solution to it.

I configure VLANs so that they are available in dom0; domUs don't know
that they use VLANs.


I use a stock "network-bridge" comming from Xen for setting up networking:


#!/bin/bash
dir=$(dirname "$0")

"$dir/network-bridge" "$@" vifnum=0 netdev=eth0 bridge=xenbr0

"$dir/network-bridge" "$@" vifnum=1 netdev=eth0.100 bridge=xenbr100

"$dir/network-bridge" "$@" vifnum=2 netdev=eth0.200 bridge=xenbr200



If the server has a 1 Gbit network card (e1000, tg3 module), VLANs work
flawlessly.

If, however, I replace the card, and use a 100 Mbit one (Intel/e100 or
Realtek/8139too module), and don't change the scripts, VLANs are not
working.


Why is it so? What is so fundamental in Xen networking that VLANs
behaviour change so much whether one uses a 1 Gbit, or 100 Mbit network
cards?


There is one difference between 1 Gbit and 100 Mbit cards - 100 Mbit
cards won't allow to set the MTU bigger than 1500:

# ifconfig eth1 mtu 1501
SIOCSIFMTU: Invalid argument


But I'm not sure if it has anything with the described VLAN problem.

On the other hand, if I configure VLANs in domU, it works fine then (but
it's a bad approach for me, and prefer to have VLANs in dom0 only).


-- 
Tomasz Chmielewski
http://wpkg.org