Interesting developments. I agree that it doesn't seem like `bridge-nf-
filter-vlan-tagged` is what we want, unless there is a special case not
filter packets tagged on VID 0. It might be worth trying this out on the
bridge used to boot the pod VMs.

The frustrating thing about this bug report is the large amount of
layers we would need to check for correct behavior. We are trying to
verify all of the following points of contact with a "VLAN 0" tagged
packet at once:

 - The network infrastructure
   (it would be better for VLAN 0 tags to be stripped before reaching end nodes)
 - NIC hardware (some NICs handle VLAN filtering in hardware; I'm not sure if 
they could automatically discard the VLAN 0 tag before it's handed up the stack)
 - Linux NIC driver (would be programming the aforementioned hardware to 
hopefully do the right thing - or not)
 - Linux bridge driver (or whatever is handing the packets from the OS to the 
pod)
 - Hypervisor NIC driver (that is, the virtual hardware which will be booting 
on the VM)
 - iPXE (which would be using a minimal network stack prior to booting the 
virtual OS)
 - Virtual OS Linux driver

I feel like we aren't sure which of these layers might have a problem
handling packets tagged on VID 0. We know iPXE has a problem for sure;
the other layers would need to be tested in isolation to verify the
behavior.

Andres confirmed that RHEL/CentOS are patching iPXE to treat VLAN 0
frames as untagged (his link showed that the patch was on CentOS 7). But
there are many layers here that could be handling them improperly. That
is, if we know that CentOS 6 works, we can't rule out that CentOS 7 may
not be working due to an issue elsewhere in the stack (not in iPXE).

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https://bugs.launchpad.net/bugs/1805920

Title:
  iPXE ignores vlan 0 traffic

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