MAAS should warn loudly when it detects mismatched MTU settings

Bug #1733945 reported by Chris Gregan on 2017-11-22
8
This bug affects 1 person
Affects Status Importance Assigned to Milestone
MAAS
Undecided
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Bug Description

We recently ran into an issue that was difficult to triage, but ended up being a mismatch in MTU settings between the vlan and the services deployed on that vlan. It would be very useful if MAAS would throw up a flashing red light if it detects that the MTU settings on a deployment it is attempting to handle are mismatched.

See bug https://bugs.launchpad.net/juju/+bug/1733592 for additional details

Changed in maas:
status: New → Incomplete
milestone: none → 2.4.x
Andres Rodriguez (andreserl) wrote :

Hi Chris,

While warning the user about the MTU difference may or may not be the solution, I'd like to understand the problem before really working on a fix. As such:

1. What do you mean by "mismatch in MTU settings between the vlan and the services deployed on that vlan." ?
 - Does that mean that the juju deployed service changed the MTU ?
 - Does that mean that MAAS model had changed the VLAN value, but controllers connected to the same VLAN have a different value set?
 - Does that mean the MAAS VLAN config one MTU value, but the configured interface in MAAS had a different MTU value?
 - What values differed? (e.g. what was the MTU value in the VLAN vs the configured one?)

2. If this is MAAS configuration, how did the configuration happen:
 - you set the MTU value on the vlan, then configured interfaces with a different MTU value?
 - Did you configure the interfaces with a MTU value, and then changed the VLAN MTU value?

3. Can you provide steps to reproduce this issue?

4. The reference to the juju bug refers to two physical interfaces with different MTU values. I'm unclear whether MAAS deployed the machines like that, and how should that account for MAAS notifying the user of different MTU's, since nothing there seems as it would be incorrect (hence, need to understand what the real issue is).

Chris Gregan (cgregan) wrote :

@jason
Can you answer these for Andres

Jason Hobbs (jason-hobbs) wrote :

It means "the MAAS VLAN config one MTU value, but the configured interface in MAAS had a different MTU value"

We configured the VLAN MTU value first, and then configured the MTU on the interface. We expect to get an error when configuring the mtu on an interface to a value other than the value that is configured to on the VLAN.

To reproduce:
1) set a VLAN in MAAS to mtu 1500.
2) configure an interface to use that vlan, and set the interface's MTU to 9000. You should see an error at this point and be prevented from setting that value, but you don't.
3) Using juju, deploy the node and a container to the node. (juju deploy ubuntu; juju deploy ubuntu --to lxd:0)
4) The machine will get an interface configured with MTU 9000, then a bridge will be created, and an interface for the container will be created with MTU 1500, lowering the MTU for the bridge/physical device to 1500.

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