Availability Zones guide for OVN

The purpose of this page is to describe how the availability zones works with OVN. Prior to reading this document, it is recommended to first read ML2/OVS driver Availability Zones guide.

There are two types of availability zones available in Neutron: Router and Network. For ML2/OVS, this is related to the scheduling of the L3 agent and DHCP agent respectively. For ML2/OVN, it’s about the scheduling of logical router ports and “external” ports respectively.

More details about each type of availability zones can be found later in this document but first let’s go over the common parts between them:

How to configure it

Different from the ML2/OVS driver for Neutron the availability zones for the OVN driver is not configured via a configuration file. Since ML2/OVN does not rely on an external agent such as the L3 agent, certain nodes (e.g gateway/networker node) won’t have any Neutron configuration file present. For this reason, OVN uses the local OVSDB for configuring the availability zones that instance of ovn-controller running on that hypervisor belongs to.

The configuration is done via the ovn-cms-options entry in external_ids column of the local Open_vSwitch table:

$ ovs-vsctl set Open_vSwitch . external-ids:ovn-cms-options="enable-chassis-as-gw,availability-zones=az-0:az-1:az-2"

The above command is adding two configurations to the ovn-cms-options option, the enable-chassis-as-gw option which tells the OVN driver that this is a gateway/networker node and the availability-zones option specifying three availability zones: az-0, az-1 and az-2.


Specifying the “enable-chassis-as-gw” option is not required for the Availability Zones however ML2/OVN will only consider nodes that are gateway (the ones with the “enable-chassis-as-gw” option) when scheduling both router and external ports. So, even tho the “availability-zones” option can be set own their own, the ML2/OVN driver does not have a use case for it at the moment.

Note that, the syntax used to specify the availability zones is the availability-zones word, followed by an equal sign (=) and a colon separated list of the availability zones that this local ovn-controller instance belongs to.

To confirm the specific ovn-controller availability zones, check the Availability Zone column in the output of the command below:

$ openstack network agent list
| ID                                   | Agent Type                   | Host           | Availability Zone | Alive | State | Binary         |
| 2d1924b2-99a4-4c6c-a4f2-0be64c0cec8c | OVN Controller Gateway agent | gateway-host-0 | az0, az1, az2     | :-)   | UP    | ovn-controller |


If you know the UUID of the agent the “openstack network agent show <UUID>” command can also be used.

To confirm the availability zones defined in the system as a whole:

$ openstack availability zone list --network
| Zone Name | Zone Status |
| az0       | available   |
| az1       | available   |
| az2       | available   |

Router Availability Zones

In order to create a router with availability zones the --availability-zone-hint should be passed to the create command, note that this parameter can be specified multiple times in case the router belongs to more than one availability zone. For example:

$ openstack router create --availability-zone-hint az-0 --availability-zone-hint az-1 router-0
| Field                   | Value                                |
| admin_state_up          | UP                                   |
| availability_zone_hints | az-0, az-1                           |
| availability_zones      |                                      |
| created_at              | 2020-06-04T08:29:33Z                 |
| description             |                                      |
| external_gateway_info   | null                                 |
| flavor_id               | None                                 |
| id                      | 8fd6d01a-57ad-4e91-a788-ebe48742d000 |
| name                    | router-0                             |
| project_id              | 2a364ced6c084888be0919450629de1c     |
| revision_number         | 1                                    |
| routes                  |                                      |
| status                  | ACTIVE                               |
| tags                    |                                      |
| updated_at              | 2020-06-04T08:29:33Z                 |

It’s also possible to set the default availability zones via the /etc/neutron/neutron.conf configuration file:

default_availability_zones = az-0,az-2

When scheduling the gateway ports of a router, the OVN driver will take into consideration the router availability zones and make sure that the ports are scheduled on the nodes belonging to those availability zones.

Note that in the router object we have two attributes related to availability zones: availability_zones and availability_zone_hints:

| availability_zone_hints | az-0, az-1                           |
| availability_zones      |                                      |

This distinction makes more sense in the ML2/OVS driver which relies on the L3 agent for its router placement (see the ML2/OVS driver Availability Zones guide for more information). In ML2/OVN the ovn-controller service will be running on all nodes of the cluster so the availability_zone_hints will always match the availability_zones attribute, as below:

| availability_zone_hints | az-0, az-1                           |
| availability_zones      | az-0, az-1                           |

OVN Database information

In order to check the availability zones of a router via the OVN Northbound database, one can look for the neutron:availability_zone_hints key in the external_ids column for its entry in the Logical_Router table:

$ ovn-nbctl list Logical_Router
_uuid               : 4df68f1e-17dd-4b9a-848d-b6152ae19203
external_ids        : {"neutron:availability_zone_hints"="az-0,az-1", "neutron:gw_port_id"="", "neutron:revision_number"="1", "neutron:router_name"=router-0}
name                : neutron-8fd6d01a-57ad-4e91-a788-ebe48742d000

To check the availability zones of the Chassis, look at the ovn-cms-options key in the other_config column (or external_ids for an older version of OVN) of the Chassis table in the OVN Southbound database:

$ ovn-sbctl list Chassis
_uuid               : abaa9f07-9988-40c0-bd1a-8d8326af08b0
name                : "2d1924b2-99a4-4c6c-a4f2-0be64c0cec8c"
other_config        : {..., ovn-cms-options="enable-chassis-as-gw,availability-zones=az-0:az-1:az-2"}

As mentioned in the Router availability zones section, the scheduling of the gateway router ports will take into consideration the availability zones that the router belongs to. We can confirm this behavior by looking in the Gateway_Chassis table from the OVN Northbound database:

$ ovn-nbctl list Gateway_Chassis
_uuid               : ac61b70f-ff51-43d9-830b-f9bc6d74090a
chassis_name        : "2d1924b2-99a4-4c6c-a4f2-0be64c0cec8c"
external_ids        : {}
name                : lrp-5a40eeca-5233-4029-a470-9018aa8b3de9_2d1924b2-99a4-4c6c-a4f2-0be64c0cec8c
options             : {}
priority            : 2

_uuid               : c1b7763b-1784-4e5a-a948-853662faeddc
chassis_name        : "1cde2542-69f9-4598-b20b-d4f68304deb0"
external_ids        : {}
name                : lrp-5a40eeca-5233-4029-a470-9018aa8b3de9_1cde2542-69f9-4598-b20b-d4f68304deb0
options             : {}
priority            : 1

Each entry on this table represents an instance of the gateway port (L3 HA, for more information see Routing in OVN), the chassis_name column indicates which Chassis that port instance is scheduled onto. If we co-relate each entry and their chassis_name we will see that this port has been only scheduled to Chassis matching with the router’s availability zones.

Network Availability Zones

Since OVN has a distributed DHCP server model (see the ovn-architecture document for more information), one may think that there’s no need for Ml2/OVN to support Network Availability Zones as there’s no need to co-locate a DHCP agent within the same zones to serve the VMs but, in ML2/OVN there’s a special case which are the external ports and those need to be aware of the Availability Zones for its scheduling.

These external ports are ports that are located on a different node than the one that the VM is running. At the moment, ML2/OVN only supports one case that makes use of these ports which is the SR-IOV support.

In order to create a network with availability zones the --availability-zone-hint should be passed to the create command, note that this parameter can be specified multiple times in case the network belongs to more than one availability zone. For example:

$ openstack network create --availability-zone-hint az-0 --availability-zone-hint az-1 network-0
| Field                     | Value                                |
| admin_state_up            | UP                                   |
| availability_zone_hints   | az-0, az-1                           |
| availability_zones        |                                      |
| created_at                | 2021-04-26T14:04:51Z                 |
| description               |                                      |
| dns_domain                |                                      |
| id                        | ba584cdb-b866-4744-85d3-6e38718055cc |
| ipv4_address_scope        | None                                 |
| ipv6_address_scope        | None                                 |
| is_default                | False                                |
| is_vlan_transparent       | None                                 |
| mtu                       | 1442                                 |
| name                      | network-0                            |
| port_security_enabled     | True                                 |
| project_id                | ffd9e4a60af34b0599f1d50aed20dde0     |
| provider:network_type     | None                                 |
| provider:physical_network | None                                 |
| provider:segmentation_id  | None                                 |
| qos_policy_id             | None                                 |
| revision_number           | 1                                    |
| router:external           | Internal                             |
| segments                  | None                                 |
| shared                    | False                                |
| status                    | ACTIVE                               |
| subnets                   |                                      |
| tags                      |                                      |
| updated_at                | 2021-04-26T14:04:52Z                 |

OVN Database information

Upon creating the first external port to a network with Availability Zones set a HA Chassis Group correspondent to that network will also be created in the OVN Northbound Database:

$ openstack port create --network network-0 --vnic-type direct port-0
| Field                   | Value                                |
| id                      | 2523d7f5-c7ca-40b8-83c5-ac37e5b126ea |
| name                    | port-0                               |
| network_id              | ba584cdb-b866-4744-85d3-6e38718055cc |

To find the corresponding HA Chassis Group we need to look for a group named as neutron-<Neutron Network UUID>, for example:

$ ovn-nbctl list HA_Chassis_Group neutron-ba584cdb-b866-4744-85d3-6e38718055cc
_uuid               : f6a49abb-dc97-4e2a-955a-6f8e8be4865e
external_ids        : {"neutron:availability_zone_hints"="az-0,az-1"}
ha_chassis          : [46850075-7383-4da9-b0b2-5ded2858f681, ce1da6a5-77d3-4945-b218-c0ae35403b80]
name                : neutron-ba584cdb-b866-4744-85d3-6e38718055cc

In the output above is possible to see that the HA Chassis Group for the Neutron network ba584cdb-b866-4744-85d3-6e38718055cc includes two Chassis (the ha_chassis column) that are part of the Availability Zones that this network is also part of.

We can inspect these members to see which one has the highest priority, which means that when the external port is bound it will first bound to the HA Chassis with the highest priority in the Group. In case that Chassis goes down the port will move on to the next Chassis with the highest priority and so on. To check these HA Chassis do:

$ ovn-nbctl list HA_Chassis 46850075-7383-4da9-b0b2-5ded2858f681
_uuid               : 46850075-7383-4da9-b0b2-5ded2858f681
chassis_name        : "2c5c4479-0e2b-4742-a1d7-df10be020143"
external_ids        : {}
priority            : 32766

$ ovn-nbctl list HA_Chassis ce1da6a5-77d3-4945-b218-c0ae35403b8
_uuid               : ce1da6a5-77d3-4945-b218-c0ae35403b80
chassis_name        : "159970f0-71f7-4d3d-9a9e-92e37c5f03c5"
external_ids        : {}
priority            : 32767

In this case, the active Chassis is the 159970f0-71f7-4d3d-9a9e-92e37c5f03c5.

And lastly, to find which HA Chassis Group an external port belongs to by looking into the OVN Northbound Database do:

$ sudo ovn-nbctl list Logical_Switch_Port 2523d7f5-c7ca-40b8-83c5-ac37e5b126ea
_uuid               : 382d8cd8-575f-4a3f-93ba-a01cb9c2c265
ha_chassis_group    : f6a49abb-dc97-4e2a-955a-6f8e8be4865e
name                : "2523d7f5-c7ca-40b8-83c5-ac37e5b126ea"
type                : external

The ha_chassis_group column will point to the UUID (in the OVN database) of the HA Chassis Group it belongs to.