[libvirt] Exact meaning of "nativeMode" attribute in vlan tags

[Dan Mossor]

On 02/22/2015 01:31 PM, Laine Stump wrote:
> You'd think that I would know this, since I'm the person who reviewed
> jrobson's patch adding support for the nativeMode attribute to the vlan
> tag element. But you'd be wrong. Here is what the config looks like:
>
>
>      <vlan trunk='yes'>
>        <tag id='42' nativeMode='untagged'/>
>        <tag id='47'/>
>      </vlan>
>
> I understand that trunk='yes' means that packets with any of the tags
> listed in a <tag> subelement can be sent out this port (and the tag will
> *not* be removed), and likewise packets arriving into the bridge from
> the port are allowed to have any of the listed tags (and, again, no tag
> will be removed). But what exactly do nativeMode='untagged' and
> nativeMode='tagged' mean?
>
> As I understand it, (nativeMode='untagged'|nativeMode='tagged') means
> that packets (arriving from|sent to) the port (without a tag/with that
> tag) will be (tagged|untagged). Can someone who fully understands this
> please select A or B for each of the 4 parenthesized items (in as many
> permutations as make sense).
>
> I guess that in one of the modes, untagged packets going in one
> direction or the other will be tagged, and vice versa, I just don't know
> which direction does which, and for which mode, and don't want to guess.
>
> (I'm asking this because I want to implement identical functionality for
> standard Linux host bridges - I want to make sure there are no surprises
> for people switching between OVS and Linux host bridge implementations).
>
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>
Laine,

You are on the right path for figuring this out. I'm coming from a more 
traditional hardware networking viewpoint with vast experience in Cisco 
and HP switching products. In that arena, the "Native" vlan is the vlan 
of the trunk itself. For example, lets say you have a trunk carrying 
vlans 20-26, with a native vlan of 15. This native vlan only comes into 
play when that vlan interface on the switch itself has an IP address. 
Consider it the infrastructure management vlan - you really don't want 
it on vlan 1 (which is the default, and all trunks are have a native 
vlan of 1) for security purposes, so you create a management vlan and 
assign the vlan interface on all of your switches with an IP address 
from that range. When the trunks are built, the native vlan carries the 
untagged packets for the management network, while the tagged vlans are 
trunked through.

In this specific scenario, and to answer your questions as asked, 
nativeMode='untagged' means that packets traveling to and from that port 
on that specific vlan ID will be untagged. nativeMode='tagged' means 
that packets in the native vlan will be tagged when sent or received on 
that port.

I hope this helps, I can try to reword it if I made it even more confusing.

-Dan

-- 
Dan Mossor
Systems Engineer at Large
Fedora KDE WG | Fedora QA Team | Fedora Server SIG
Fedora Infrastructure Apprentice
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San Antonio, Texas, USA