Use of OVN Networking

Overview

OVN is largely considered an evolution of OVS. While it is recommended that operators continue to utilize OVS with Ironic, OVN has an attractive a superset of capabilities and shifts some of the configuration of networking away from configuration files, towards a service modeled at serving a more scalable software defined networking experience. However, as with all newer technologies, there are caveats and issues. The purpose of this documentation is to help convey OVN’s state, capabilities, and provide operators with the context required to navigate their path forward.

Warning

OVN is under quite a bit of active development, and this information may grow out of date quickly. We’ve provided links to help spread the information and enable operators to learn the current status.

Challenges

DHCP

Historically, while OVN has included a DHCP server, this DHCP server has not had the capability to handle clients needing custom attributes such as those used by PXE and iPXE to enable network boot operations.

Typically, this has resulted in operators who use OVN with Bare Metal to continue to operate the neutron-dhcp-agent service, along with setting OVN configuration appropriate to disable OVN from responding to DHCP requests for baremetal ports. Please see routed networks for more information on this setting.

As of the 2023.2 Release of Ironic, The Ironic project can confirm that OVN’s DHCP server does work for PXE and iPXE operations when using IPv4, OVS version 3.11, and OVN version 23.06.0.

Support for IPv6 is presently pending changes to Neutron, as IPv6 requires additional configuration options and a different pattern of behavior, and thus has not been tested. Your advised to continue to us the neutron-dhcp-agent if you need IPv6 at this time. Currently this support is being worked in Neutron change 890683 and bug 20305201.

Warning

Use of OVN with HTTPBoot interfaces has not been explicitly tested by the Ironic project, and is unlikely to take place until after integrated IPv6 support with Neutron is ready for use. The project does not expect any specific issues, but the OVN DHCP server is an entirely different server than the interfaces were tested upon.

Maxmium Transmission Units

OVN’s handling of MTUs has been identified by OVN as being incomplete. The reality is that it assumes the MTU is not further constained beyond the gateway, which sort of works in some caess for virtual machines, but might not be applicable with baremetal because your traffic may pass through lower, or higher MTUs.

Ideally, your environment should have consistent MTUs. If you cannot have consistent MTUs, we recommend clamping the MTU and Maximum Segment Size (MSS) using your front end router to ensure igress traffic is sized and fragmented appropriately. Egress traffic should inherent it’s MTU size based upon the DHCP service configuration.

A items you can keep track of regarding MTU handling:

To clamp the MTU and MSS on a linux based router, you can utilize the following command:

ip route add $network via $OVN_ROUTER advmss $MAX_SEGMENT_SIZE mtu lock $MTU

NAT of TFTP

Because the NAT and Connection Tracking layer gets applied differently with OVN, as the router doesn’t appear as a namespace or to the local OS kernel, you will not be able to enable NAT translation for Bare Metal Networks under the direct management of OVN, that is if you don’t have a separate TFTP service running from with-in that network.

This is a result of the kernel of the OVN gateway being unable to associate and handle return packet directly as part of the connection tracking layer. No direct work around for this is known, but generally Ironic encourages the use of Virtual Media where possible to sidestep this sort of issue and ensure a higher operational security posture for the deployment. Users of the redfish hardware type can learn about Virtual media boot in our Redfish documentation.

Warning

Creation of FIPs, such as those which may be used grant SSH access to a internal node on a network, for example which may be used by Tempest, establishes a 1:1 NAT rule. When this is the case, TFTP packets cannot transit OVN and network boot operations will fail.

Rescue

Due to the aformentioned NAT issues, we know Rescue operations may not work.

This is being tracked as bug 2033083.

PXE boot of GRUB

Initial testing has revelaed that EFI booting Grub2 via OVN does not appear to work with OVN. For some reason, Grub2 believes the network mask is incorrect based upon the DHCP interaction, and results in a belief that the TFTP server is locally attached.

For example, if a client is assigned 10.1.0.13/28, with a default gateway of 10.1.0.1, and a tftp-sever of 10.203.101.230, then grub2 believes it’s default route is 10.0.0.0/8.

This is being tracked as bug 2033430 until we’re better able to understand the root cause and file a bug with the appropriate project.

Required Configuration

OVN is designed to provide packet handling in a distributed fashion for a each compute hypervisor in a cloud of virtual machines. However with Bare Metal instances, you will likely need to have a pool of dedicated “network nodes” to handle OVN traffic.

Chassis as Gateway

The networking node chassis must be configured to operate as a gateway.

This can be configured manually, but should (as far as Ironic is aware) be configured by Neutron and set on interfaces matching the bridge mappings. At least, it works that way in Devstack.

ML2 Plugins

The ovn-router and trunk ml2 plugins as supplied with Neutron must be enabled.

If you need to attach to the network…

For example if you need to bind something into a network for baremetal, above and beyond a dedicated interface, you will need to make the attachment on the br-ex integration bridge, as opposed to br-int as one would have done with OVS.

VTEP Switch Support

Alpha-quality support was added to Ironic for OVN VTEP switches in API version 1.90. When the keys vtep-logical-switch, vtep-physical-switch, and port_id are set in port.local_link_connection, Ironic will pass them on to Neutron to be included in the binding profile to enable OVN support.

There are reports of this approach working, but Ironic developers do not have access to physical hardware to fully test this feature. If you have any feedback for this feature, please reach out to the Ironic community.

Unknowns

It is presently unknown if it is possible for OVN to perform and enable VXLAN attachments to physical ports on integrated devices, thus operators are advised to continue to use vlan networking with their hosts with existing ML2 integrations.