Hello, this is a follow up of a post on xen-users I realize this post will probably be more adequate on this list. I have a small problem; Is there a known limitation on vif code - Network-device backend or frontend - which prohibits the usage of tagged (802.1q) frames inside a domU ? In dom0 I have a bridge with eth0 (epro1000) wich is connected to a 802.1q port this same bridge has also a vif connected, corresponding to a domU. Inside this domU , I can use vconfig to have numerous interfaces derived from the vif, one by vlan, BUT can only safely use a MTU of 1496. If I use the default MTU I have problems. For the moment I have solved the problem by using the physicall interface INSIDE the domU. But I''d like to know if this problem is known ? -- Yann Dupont, Cri de l''université de Nantes Tel: 02.51.12.53.91 - Fax: 02.51.12.58.60 - Yann.Dupont@univ-nantes.fr _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
> I have a small problem; Is there a known limitation on vif > code - Network-device backend or frontend - which prohibits > the usage of tagged > (802.1q) frames inside a domU ? > > In dom0 I have a bridge with eth0 (epro1000) wich is > connected to a 802.1q port this same bridge has also a vif > connected, corresponding to a domU. > > Inside this domU , I can use vconfig to have numerous > interfaces derived from the vif, one by vlan, BUT can only > safely use a MTU of 1496. If I use the default MTU I have problems. > > For the moment I have solved the problem by using the > physicall interface INSIDE the domU. But I''d like to know if > this problem is known ?The frontend/backend itself should work for any MTU up to something shy of 4KB. I suspect the 1496 restriction is actually being introduced in dom0 by the 802.1q tagging, but this information is not being propagated to the frontend in the domU. Because this is a bridged rather than routed setup, MTU discovery won''t help. I''ve a feeling that an ''mtu='' parameter might have been added a while back so that you could fix the MTU of a frontend vif in the domain config file. We could figure this out automatically by looking at the bridge the vif is connected to and then pick the lowest MTU of any device connected to the bridge. Patches welcome. Ian _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Yann Dupont
2005-Jul-22 12:30 UTC
Re: [Xen-devel] vif limitation to 1500 bytes (no vlan ?)
Ian Pratt a écrit :> > >>I have a small problem; Is there a known limitation on vif >>code - Network-device backend or frontend - which prohibits >>the usage of tagged >>(802.1q) frames inside a domU ? >> >>In dom0 I have a bridge with eth0 (epro1000) wich is >>connected to a 802.1q port this same bridge has also a vif >>connected, corresponding to a domU. >> >>Inside this domU , I can use vconfig to have numerous >>interfaces derived from the vif, one by vlan, BUT can only >>safely use a MTU of 1496. If I use the default MTU I have problems. >> >>For the moment I have solved the problem by using the >>physicall interface INSIDE the domU. But I''d like to know if >>this problem is known ? >> >> > >The frontend/backend itself should work for any MTU up to something shy >of 4KB. > >ok, good to know,>I suspect the 1496 restriction is actually being introduced in dom0 by >the 802.1q tagging, but this information is not being propagated to the >frontend in the domU. >Because this is a bridged rather than routed setup, MTU discovery won''t >help. > >yes>I''ve a feeling that an ''mtu='' parameter might have been added a while >back so that you could fix the MTU of a frontend vif in the domain > >ah , that''s good news. I''ll go searching for that...>config file. We could figure this out automatically by looking at the >bridge the vif is connected to and then pick the lowest MTU of any > > >device connected to the bridge. Patches welcome. > >A manual way to do this is already a good start... thanks for your answer. -- Yann Dupont, Cri de l''université de Nantes Tel: 02.51.12.53.91 - Fax: 02.51.12.58.60 - Yann.Dupont@univ-nantes.fr _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel