[RFC PATCH] doc: clarify VLAN and QinQ stripping behaviour

[Bruce Richardson]

On Wed, Jul 16, 2025 at 03:24:55PM -0400, Dean Marx wrote:
> I've created a list of test cases which I'm planning on implementing
> in future QinQ testing. After talking with Bruce about some Intel test
> results earlier, I've got a solid idea of what the expected behaviors
> are in five of these, but I'm unsure about the other three. If anyone
> is able to provide some insight as to what they believe these expected
> behaviors should be, please let me know as it would be immensely
> helpful. Thanks!
> 
> Vlan Strip ON and QinQ Strip OFF:
> 1. Single VLAN:
>     Expected result: VLAN Stripped
> 2. Double VLAN:
>     Expected result: Should strip outer VLAN
> 3. Single 0x88a8 tag only:
>     Expected result: 0x88a8 tag stripped
> 4. Single VLAN + 1 0x88a8 tag:
>     Expected result: 0x88a8 tag stripped, inner VLAN untouched

Technically, the result goes for the test case of "0x88a8 tag +
Single VLAN", since the 88a8 tag is first. :-)
The "Single VLAN + 0x88a8 Tag" would be a 5th test case if you
want to try it. Expected result should be again that outer tag is stripped.

As for the expected results you quote, those may or may not be expected
depending on whether hardware supports multiple VLAN tag eth-types or
configurable VLAN tag eth-types. From my testing, in at least some of our
NICs by default, they ONLY strip VLAN tags if they have a 0x8100 tag.
However, I would be surprised if that is not configurable.

> 
> Vlan Strip ON and QinQ Strip ON:
> 1. Single VLAN:
>     Expected result: VLAN stripped
> 2. Double VLAN:
>     Expected result: Unknown (Outer VLAN stripped?)
> 3. Single 0x88a8 tag only:
>     Expected result: Unknown (0x88a8 tag stripped?)
> 4. Single VLAN + 1 0x88a8 tag:
>     Expected result: Unknown (0x88a8 tag stripped and VLAN tag stripped?)

Again, the expected result would likely depend on what are configured
ethtypes for recognised VLANs.

/Bruce