OVN External Ports

The purpose of this page is to describe how ML2/OVN leverages the use of OVN’s external ports feature.

What is it

The external ports feature in OVN allows for setting up a port that lives externally to the instance and is reponsible for replying to ARP requests (DHCP, internal DNS, IPv6 router solicitation requests, etc…) on its behalf. At the moment this feature is used in two use cases for ML2/OVN:

  1. SR-IOV

  2. Baremetal provisioning

ML2/OVN will create a port of the type external for ports with the following VNICs:

  • direct

  • direct-physical

  • macvtap

  • baremetal

Also, ports of the type external will be scheduled on the gateway nodes (controller or networker nodes) in HA mode by the OVN Neutron driver. Check the OVN Database information section for more information.

OVN Database information

the ML2/OVN driver identifies a gateway node by the ovn-cms-options=enable-chassis-as-gw and ovn-bridge-mappings options in the external_ids column from the Chassis table in the OVN Southbound database:

$ ovn-sbctl list Chassis
_uuid               : 12b13aff-a821-4cde-a4ac-d9cf8e2c91bc
external_ids        : {ovn-cms-options=enable-chassis-as-gw, ovn-bridge-mappings="public:br-ex", ...}
hostname            : controller-0
name                : "1a462946-ccfd-46a6-8abf-9dca9eb558fb"

For more information about both of these options, please take a look at the ovn-controller documentation.

These options can be set by running the following command locally on each gateway node (note, the ovn-bridge-mappings will need to be adapted to your environment):

$ ovs-vsctl set Open_vSwitch . external-ids:ovn-cms-options=\"enable-chassis-as-gw\" external-ids:ovn-bridge-mappings=\"public:br-ex\"

As mentioned in the What is it section, every time a Neutron port with a certain VNIC is created the OVN driver will create a port of the type external in the OVN Northbound database. These ports can be found by issuing the following command:

$ ovn-nbctl find Logical_Switch_Port type=external
_uuid               : 105e83ae-252d-401b-a1a7-8d28ec28a359
ha_chassis_group    : [43047e7b-4c78-4984-9788-6263fcc69885]
type                : external

The ha_chassis_group column indicates which HA Chassis Group that port belongs to, to find that group do:

# The UUID is the one from the ha_chassis_group column from
# the Logical_Switch_Port table
$ ovn-nbctl list HA_Chassis_Group 43047e7b-4c78-4984-9788-6263fcc69885
_uuid               : 43047e7b-4c78-4984-9788-6263fcc69885
external_ids        : {}
ha_chassis          : [3005bf84-fc95-4361-866d-bfa1c980adc8, 72c7671e-dd48-4100-9741-c47221672961]
name                : neutron-4b2944ca-c7a3-4cf6-a9c8-6aa541a20535


The external ports will be placed on a HA Chassis Group for the network that the port belongs to. Those HA Chassis Groups are named as neutron-<Neutron Network UUID>, as seeing in the output above. You can also use this “name” with the ovn-nbctl list command when searching for a specific HA Chassis Group.

The chassis that are members of the HA Chassis Group are listed in the ha_chassis column. Those are the gateway nodes (controller or networker nodes) in the deployment and it’s where the external ports will be scheduled. In order to find which gateway node the external ports are scheduled on use the following command:

# The UUIDs are the UUID members of the HA Chassis Group
# (ha_chassis column from the HA_Chassis_Group table)
$ ovn-nbctl list HA_Chassis 3005bf84-fc95-4361-866d-bfa1c980adc8 72c7671e-dd48-4100-9741-c47221672961
_uuid               : 3005bf84-fc95-4361-866d-bfa1c980adc8
chassis_name        : "1a462946-ccfd-46a6-8abf-9dca9eb558fb"
external_ids        : {}
priority            : 32767

_uuid               : 72c7671e-dd48-4100-9741-c47221672961
chassis_name        : "a0cb9d55-a6da-4f84-857f-d4b674088c8c"
external_ids        : {}
priority            : 32766

Note the priority column from the previous command, the chassis with the highest priority from that list is the chassis that will have the external ports scheduled on it. In our example above, the chassis with the UUID 1a462946-ccfd-46a6-8abf-9dca9eb558fb is the one.

Whenever the chassis with the highest priority goes down, the ports will be automatically scheduled on the next chassis with the highest priority which is alive. So, the external ports are HA out of the box.