Issues with default instance MTU of 1500 on OVN networks

Incus
1

HI

Most of my Windows VM instances on OVN networks are getting MTU of 1500 and that’s causing problems with a number of things, one e.g.: can’t even get to https://portal.azure.com.

If I manually set the MTU to 1442 in the VM then things work again.

I’ve tried recreating the network and launching a new instance on it but they still get 1500. Here is an example of an OVN network:

config:
  bridge.mtu: "1442"
  dns.domain: subdomain1.domain.tld
  dns.search: subdomain1.domain.tld,subdomain2.domain.tld
  ipv4.address: 10.234.123.1/24
  ipv4.dhcp: "true"
  ipv4.nat: "false"
  ipv6.address: none
  ipv6.dhcp: "false"
  ipv6.nat: "false"
  network: somecluster-inu01
  volatile.network.ipv4.address: 10.0.1.2
description: ""
name: some-ovn-network01
type: ovn
used_by:
- /1.0/profiles/someprofile_ovn?project=some-project
managed: true
status: Created
locations:
- clustermember1
- clustermember2
- clustermember3
- clustermember4
- clustermember5
- clustermember6

Can Incus’ integration with OVN networks (managed entirely via Incus), provide MTU settings via DHCP to clients?

Any ideas to avoid having set the MTU at the Incus / OVN level so that instances get automatically configured?

Thanks

2

Can you file an issue at GitHub · Where software is built?

I just checked and we seem to have logic to handle that for containers but not so much for VMs, so we should take a look at what’s going on there, whether it’s the property never making it into DHCP or the instance ignoring it for some reason or some conflict between the IPv6 RA and IPv4 DHCP or something.

3

Hi, Thanks for your quick response.

I’ve created : Some TLS site/API failures fixed by manually dropping instance MTU to 1442 when on OVN · Issue #1675 · lxc/incus · GitHub

It looks like the problem I think I have is more complex than I realise and that DHCP service might not be the right place to attack this, after reading:

All I can tell you is that Windows Server 2022 VMs work fine with MTU of 1500 on macvlan but the same instances don’t work too well on OVN unless I manually drop its MTU to 1442.

4

Yeah, if the MTU is incorrect, you’re definitely going to have some issues.

We’ll make sure that OVN correctly advertises the MTU on both the IPv6 RA and DHCP responses. If it does, then there’s not much else we can do if the OS doesn’t request/respond to that information.

5

Good morning.

I’ve launched the following on an OVN network, so it’s just Windows VMs that have this issue.

  1. ubuntu noble container mtu 1442
  2. ubuntu noble desktop vm mtu 1442
6

Just wondering whether we should have MTU of 1442 on OVN, or am I missing some config somewhere that allows default MTU of 1500 on OVN networks?

Thanks

7

OVN calculates the MTU based on the MTU of the network carrying the tunnel traffic.

If you have a jumbo frame MTU on the network carrying the tunnel traffic (typically the host’s own network), then you’ll get a 1500 MTU.

1 Like
8

Okay, so that confirms that it’s not a problem on our end but is a Windows issue then…
I guess you can manually set the MTU in Windows.

9

Thanks for you help and time. Sorry about this but at least this will help someone else.