Hi Devin,

Below is one example of creating dhcp options and associating them with logical ports using ovn-nbctl commands. Please see the links shared by Lance for more details.

Lets say you have a network with cidr - 10.0.0.0/24.

One example would be

$ovn-nbctl dhcp-options-create 10.0.0.0/24

Run the command ovn-nbctl dhcp-options-list and store the uuid of it in any variable (DHCP_UUID)

Now create the dhcp options for this DHCP_UUID just created.

There are 4 dhcp options which needs to be defined (they are mandatory)

- server_id

- server_mac

- router

- lease_time

since you want to add mtu option as well, you can add the dhcp options as

$ovn-nbctl dhcp-options-set-options $DHCP_UUID server_id=10.0.0.1 server_mac=00:00:00:00:00:10 router=10.0.0.1 lease_time=3600 mtu=1400

The above is just an example. You can see the dhcp options set by running

$ovn-nbctl dhcp-options-get-options $DHCP_UUID

The dhcp options defined here have no value unless you associate these with the logical switch port.

$ovn-nbctl lsp-set-dhcpv4-options $LPORT_NAME $DHCP_UUID

Suppose if you have 3 logical ports - lp1, lp2 and lp3, you can associate it as

$ovn-nbctl lsp-set-dhcpv4-options lp1 $DHCP_UUID

$ovn-nbctl lsp-set-dhcpv4-options lp2 $DHCP_UUID

$ovn-nbctl lsp-set-dhcpv4-options lp3 $DHCP_UUID

Please let us know if you have any more questions.

You can refer to this blog to get some more inner details of how native DHCP is supported in ovn - https://numansiddiqueblog.wordpress.com/2016/08/09/native-dhcp-support-in-ovn/

Thanks

Numan

On Mon, Dec 12, 2016 at 7:28 PM, Lance Richardson <lrichard@redhat.com> wrote:

Hi Devin,

This blog posting does a good job of explaining how to configure OVN
DHCP support:

http://blog.spinhirne.com/2016/09/an-introduction-to-ovn-routing.html

The ovn-nb man page lists the DHCP options that can be provided, including
mtu:

http://openvswitch.org/support/dist-docs/

ovn-

nb.5.html

And the ovn-nbctl man page has details about the command-line interface
for setting DHCP options:

http://openvswitch.org/support/dist-docs/ovn-nbctl.8.html

I have very little experience using OVN's DHCP support, I've copied Numan
in case I've left anything out.

Lance
----- Original Message -----
> From: "Marcin Mirecki" <mmirecki@redhat.com>
> To: "Devin Acosta" <devin@pabstatencio.com>
> Cc: "users" <Users@ovirt.org>, "Lance Richardson" <lrichard@redhat.com>
> Sent: Monday, December 12, 2016 4:35:51 AM
> Subject: Re: oVirt / OVN / MTU
>
> Devin,
>
> oVirt does not currently support changing external network mtu from within
> ovirt (it rather relies on the provider handling this internally).
>
> If you are using OVN DHCP (have subnets defined for a network), you can
> modify the OVN DHCP options directly in the OVN database.
> I have never actually tested this myself, but looking at the OVN
> documentation, it should do the job on the ports.
>
> The standard OVN way to do so is to use the "ovn-vsctl set DHCP_Options ..."
> command.
> (Unfortunately as I am trying it now it tells me that modifying DHCP_Options
> is not supported)
> Alternatively, you can use the OVS python API (let me know if you need any
> help on this).
>
> Lance,
> Would changing the dhcp:options:mtu suffice?
> Could you please comment on how to modify the DHCP MTU using the OVN cmd
> line?
>
> Thanks,
> Marcin
>
>
>
> ----- Original Message -----
> > From: "Devin Acosta" <devin@pabstatencio.com>
> > To: "Marcin Mirecki" <mmirecki@redhat.com>, "users" <Users@ovirt.org>,
> > "Lance Richardson" <lrichard@redhat.com>
> > Sent: Monday, December 12, 2016 1:20:59 AM
> > Subject: Fwd: oVirt / OVN / MTU
> >
> > Marcin / Lance,
> >
> > Not sure if the list was working correctly, I couldn't see that my message
> > below made it to the list. If I need to change the MTU settings for OVN /
> > OpenVSwitch to something lower than 1500, what is the best way to do this?
> > We noticed that some instances (ie: Windows 2012R2) are having issues with
> > the default MTU of 1500, I think there is an issue at the upper layers, and
> > we can get it to work if we manually set the MTU on the instance to say
> > 1400. Is there an easy way to do this so that any VM's that come up
> > automatically get MTU of 1400?
> >
> > Devin
> >
> > ---------- Forwarded message ----------
> > From: Devin Acosta <devin@pabstatencio.com>
> > Date: Fri, Dec 9, 2016 at 2:02 PM
> > Subject: oVirt / OVN / MTU
> > To: users <Users@ovirt.org>
> >
> >
> >
> > We are running oVirt 4.0.5 and we have OVN working to provide a Virtual
> > Layer 2 network. We are noticing that because the OVN is using Geneve and
> > between all the firewalls and networks it crosses we are running into an
> > MTU issue. What is the best suggested way to lower say the entire OVN
> > network to say MTU of 1400, and also allow for fragmenting packets?
> >
> >
> > --
> >
> > Devin Acosta
> > Red Hat Certified Architect, LinuxStack
> > 602-354-1220 || devin@linuxguru.co
> >
> >
> >
> > --
> >
> > Devin Acosta
> > Red Hat Certified Architect, LinuxStack
> > 602-354-1220 || devin@linuxguru.co
> >
>