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
> >
>