[Bug 1805920] Re: iPXE ignores vlan 0 traffic

Just to clarify the above statements as it has been source of confusion.

MAAS 2.3+ (which is the latest available in Xenial), no longer uses nor
supports iSCSI. While the option to fallback to old behavior does exist,
it is not enabled by default, its obscured and, given that is not
supported, it is to be used at users risk.

That said, I'm not sure whether this change should be backported all the
way to Xenial. It would seem to be that it should be backported to
Bionic only.

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

Title:
iPXE ignores vlan 0 traffic

Status in MAAS:
Invalid
Status in ipxe package in Ubuntu:
Confirmed
Status in linux package in Ubuntu:
Confirmed

Bug description:
I have three MAAS rack/region nodes which are blades in a Cisco UCS
chassis. This is an FCE deployment where MAAS has two DHCP servers,
infra1 is the primary and infra3 is the secondary. The pod VMs on
infra1 and infra3 PXE boot fine but the pod VMs on infra2 fail to PXE
boot. If I reconfigure the subnet to provide DHCP on infra2 (either as
primary or secondary) then the pod VMs on infra2 will PXE boot but the
pod VMs on the demoted infra node (that no longer serves DHCP) now
fail to PXE boot.

While commissioning a pod VM on infra2 I captured network traffic with
tcpdump on the vnet interface.

Here is the dump when the PXE boot fails (no dhcp server on infra2):
https://pastebin.canonical.com/p/THW2gTSv4S/

Here is the dump when PXE boot succeeds (when infra2 is serving dhcp):
https://pastebin.canonical.com/p/HH3XvZtTGG/

The only difference I can see is that in the unsuccessful scenario,
the reply is an 802.1q packet -- it's got a vlan tag for vlan 0.
Normally vlan 0 traffic is passed as if it is not tagged and indeed, I
can ping between the blades with no problem. Outgoing packets are
untagged but incoming packets are tagged vlan 0 -- but the ping works.
It seems vlan 0 is used as a part of 802.1p to set priority of
packets. This is separate from vlan, it just happens to use that
ethertype to do the priority tagging.

Someone confirmed to me that, in the iPXE source, it drops all packets
if they are vlan tagged.

The customer is unable to figure out why the packets between blades is
getting vlan tagged so we either need to figure out how to allow iPXE
to accept vlan 0 or the customer will need to use different equipment
for the MAAS nodes.

I found a conversation on the ipxe-devel mailing list that suggested a
commit was submitted and signed off but that was from 2016 so I'm not
sure what became of it. Notable messages in the thread:

http://lists.ipxe.org/pipermail/ipxe-devel/2016-April/004916.html
http://lists.ipxe.org/pipermail/ipxe-devel/2016-July/005099.html

Would it be possible to install a local patch as part of the FCE
deployment? I suspect the patch(es) mentioned in the above thread
would require some modification to apply properly.

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