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Re: [libvirt] openvswitch vlan support when not a forwarded network



Thank you for your comprehensive reply!

On Tue, Sep 17, 2013 at 4:15 AM, Laine Stump <laine laine org> wrote:
> On 09/16/2013 07:34 PM, Ajith Antony wrote:
>> The resulting ephemeral bridge(virbr1) looks like the following when my
>> network(w/o vlans) and two domains are started. I don't know if the portgroup
>> was meaningful, but it was accepted in the definition:
>>
>>     $ sudo ovs-vsctl show
>>     <...>
>>         Bridge "virbr1"
>>             Port "vnet23"
>>                 Interface "vnet23"
>>             Port "vnet25"
>>                 Interface "vnet25"
>>             Port "virbr1"
>>                 Interface "virbr1"
>>                     type: internal
>>             Port "virbr1-nic"
>>                 Interface "virbr1-nic"
>>         ovs_version: "1.9.3"
>
>
> You apparently have openvswitch's "Linux host bridge compatibility"
> package installed on your machine. If you didn't, the network definition
> you have would have created a Linux host bridge rather than an
> openvswitch bridge. libvirt doesn't contain any code that can create an
> openvswitch bridge directly, so that's the only possible way this could
> be happening. The problem is that when you use compatibility mode,
> you're limited to the Linux bridge-utils API, which has no method of
> specifying a vlan tag for individual ports (because Linux host bridges
> lack that capability).

Aha!  I did not understand this.   I was under the impression that libvirt was
managing this.  I understand now.

>> Ultimately my goal is to prepare isolated test environments that consist of
>> several VM's attached to a similar qty of vlans.  I intend to create many of
>> these environments per host.  I also recongnize that instead of portgroups, I
>> could use separate networks altogether. From an administrative standpoint, I'd
>> prefer to have one "network" per test environment, with several portgroups,
>> instead of *many* networks.
>
>
> Since this is all just numbers in memory (no real cables / switches),
> there is little to no practical difference between having a single
> bridge with lots of vlans, or having lots of bridges with no vlans.
>
> One big difference is that you can do the latter today with existing
> libvirt code (and you don't even need to have openvswitch installed on
> your host). Unless you have > 255 guests on a single vlan, or need some
> other openvswitch-specific feature not available with Linux host
> bridges, I would just setup multiple networks and use the existing
> libvirt networks.

Yes, I'll probably go with the regular bridge behavior for now.   One very
attractive feature of using openvswitch is the ability to "re-wire" the whole
set-up by reassigning the vlan tags on-the-fly.  My base usecase should be
consistent with the libvirt workflow, where things like changing domain
interface configs take effect when a domain is destroyed and started again, but
the opportunity to move interfaces around without a hard power-cycle could
prove valuable.


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