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

[Robson, James]

To clarify the behaviour of open vswitch:
With either of the native modes selected, a packet that comes in to the switch without a vlan header will be placed in the native vlan and the header added before the packet is forwarded.
With nativeMode='tagged', a packet in the native vlan will be sent out of the switch with its vlan header intact. With nativeMode='untagged' a packet in the native vlan will be sent out of the switch with the vlan header removed.

In case it helps, here is the open vswitch doc for the vlan settings:
 Bridge ports support the following types of VLAN configuration:

              trunk  A  trunk  port  carries packets on one or more specified VLANs specified in the trunks column (often, on every VLAN).  A
                     packet that ingresses on a trunk port is in the VLAN specified in its 802.1Q header, or VLAN 0  if  the  packet  has  no
                     802.1Q header.  A packet that egresses through a trunk port will have an 802.1Q header if it has a nonzero VLAN ID.

                     Any packet that ingresses on a trunk port tagged with a VLAN that the port does not trunk is dropped.

              access An  access  port  carries  packets on exactly one VLAN specified in the tag column.  Packets egressing on an access port
                     have no 802.1Q header.

                     Any packet with an 802.1Q header with a nonzero VLAN ID that ingresses on an  access  port  is  dropped,  regardless  of
                     whether the VLAN ID in the header is the access port’s VLAN ID.

              native-tagged
                     A native-tagged port resembles a trunk port, with the exception that a packet without an 802.1Q header that ingresses on
                     a native-tagged port is in the ‘‘native VLAN’’ (specified in the tag column).

              native-untagged
                     A native-untagged port resembles a native-tagged port, with the exception that a  packet  that  egresses  on  a  native-
                     untagged port in the native VLAN will not have an 802.1Q header.


________________________________________
From: sendmail [justsendmailnothingelse at gmail.com] on behalf of Laine Stump [laine at laine.org]
Sent: 22 February 2015 19:31
To: Libvirt
Cc: Robson, James
Subject: Exact meaning of "nativeMode" attribute in vlan tags

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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