Gaps from ML2/OVS

This is a list of some of the currently known gaps between ML2/OVS and OVN. It is not a complete list, but is enough to be used as a starting point for implementors working on closing these gaps. A TODO list for OVN is located at 1.

  • BGP support

    Neutron-dynamic-routing supports making a tenant subnet routable via BGP, and can announce host routes for both floating and fixed IP addresses. These functions are not supported in OVN.

  • Baremetal provisioning with iPXE without Neutron DHCP agent for IPv6

    The core OVN built-in DHCP server implementation does not yet support PXE booting for IPv6. This can be achieved at the moment if used with the Neutron DHCP agent by deploying it on OVN gateway nodes and disabling the OVN DHCP by setting the [ovn]/disable_ovn_dhcp_for_baremetal_ports configuration option to True.

  • QoS minimum bandwidth allocation in Placement API

    ML2/OVN integration with the Nova placement API to provide guaranteed minimum bandwidth for ports 2. Work in progress, see 3

  • IPv6 Prefix Delegation

    Currently ML2/OVN doesn’t implement IPv6 prefix delegation. OVN logical routers have this capability implemented in 4 and we have an open RFE to fill this gap 5.

  • East/West Fragmentation

    The core OVN implementation does not support east/west fragmentation. There is no known production use-case for this feature hence we don’t even have an RFE open for it and it’s not on the roadmap to be implemented.

  • DHCP service for instances

    ML2/OVS adds packet filtering rules to every instance that allow DHCP queries from instances to reach the DHCP agent. For OVN this traffic has to be explicitly allowed by security group rules attached to the instance. Note that the default security group does allow all outgoing traffic, so this only becomes relevant when using custom security groups 6. Proposed patch is 7 but it needs to be revived and updated.

  • DNS resolution for instances

    OVN cannot use the host’s networking for DNS resolution, so Case 2b in 8 can only be used when additional DHCP agents are deployed. For Case 2a a different configuration option has to be used in ml2_conf.ini:

    [ovn]
    dns_servers = 203.0.113.8, 198.51.100.53
    
  • IPv6 NDP proxy

    The NDP proxy functionality for IPv6 addresses is not supported by OVN.

References

1

https://github.com/ovn-org/ovn/blob/master/TODO.rst

2

https://specs.openstack.org/openstack/neutron-specs/specs/rocky/minimum-bandwidth-allocation-placement-api.html

3

https://review.opendev.org/c/openstack/neutron/+/786478

4

https://patchwork.ozlabs.org/project/openvswitch/patch/6aec0fb280f610a2083fbb6c61e251b1d237b21f.1576840560.git.lorenzo.bianconi@redhat.com/

5

https://bugs.launchpad.net/neutron/+bug/1895972

6

https://bugs.launchpad.net/neutron/+bug/1926515

7

https://review.opendev.org/c/openstack/neutron/+/788594

8

https://docs.openstack.org/neutron/latest/admin/config-dns-res.html