Re: nfs, tap & vlan issues

[Fabio Coatti <cova@ferrara.linux.it>]

giovedì 18 settembre 2008, Javier Guerra ha scritto:
> On Thu, Sep 18, 2008 at 11:47 AM, Fabio Coatti <cova@ferrara.linux.it> 
wrote:
> > The network on guest machine is set up like this:
> > 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue
> >    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
> >    inet 127.0.0.1/8 brd 127.255.255.255 scope host lo
> > 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen
> > 1000 link/ether de:ad:be:ef:15:05 brd ff:ff:ff:ff:ff:ff
> > 3: vlan3@eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue
> >    link/ether de:ad:be:ef:15:05 brd ff:ff:ff:ff:ff:ff
> >    inet 192.168.0.5/24 brd 192.168.61.255 scope global vlan3
> > 4: vlan4@eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue
> >    link/ether de:ad:be:ef:15:05 brd ff:ff:ff:ff:ff:ff
> >    inet 10.0.0.33/24 brd 10.0.0.255 scope global vlan4
>
> there's your problem:
>
> your vlan interfaces (vlan3@eth0, vlan4@eth0) have an MTU of 1500. to
> encapsulate that in eth0, it has to add 4 bytes of tagging, therefore
> eth0 should have an MTU of 1504.  also, the bridge and eth1 on Dom0
> must have MTUs of 1504.
>
> i don't know if the bridge can support 1504, if not, you would have to
> set eth0 at 1500, and the vlan interfaces at 1496

I see your point; I've tried right now... and it worked! Thanks for your help, 
really greatly appreciated.
Anyway, a small detail:

At first, I've tried to bring host interfaces (eth1,kvmbr0 and tap0) up to 
1504, no problem. Then I've started guest machine and tried to raise eth0 to 
1504, but it seems that virtio driver refuses mtu bigger than 1500.

So the only difference with your recipe is that I've left all host interfaces 
(physical, tap and bridge) to an MTU of 1500 and set, on guest, eth0 to 1500, 
vlans to 1496.

So it was eth0 on guest to complain and not bridge, but now it's working like 
a charm.
(I was sure to have tried even the combination you suggested, in all my tries, 
but obvously I was wrong)


-- 
Fabio "Cova" Coatti    http://members.ferrara.linux.it/cova     
Ferrara Linux Users Group           http://ferrara.linux.it
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