Add availability zones for agents

Implement availability zones for the DHCP and L3 agents. Just like Nova and Cinder, this allows users to specify where the network services run, giving better fault isolation.

Problem Description

Nova and Cinder have availability zones today. Cloud administrators can assign availability zones to physical nodes. Each of the nodes generally is equipped different power sockets, network switches, cooling devices and others. By properly choosing from the provided availability zones, users can minimize their chance of service failures.

But, as Neutron doesn’t have availability zones, there’s no way to put network services under distinct availability zones as a VM instance or a VM volume. What happens with Neutron is completely by chance today. A user has risk of higher probability of network failure because the user cannot allocate network resources to availability zones for high availability. Also, network traffic can go through long paths between availability zones. DVR and L3 HA can mitigate these issues somewhat, but they don’t entirely solve the problem since DVR still need central SNAT router, which need to be HA capable, and L3 HA is not aware of underlying hardware configuration to be HA as a system (i.e. not only assigning routers to “other node” but to the “appropriate node (or group of nodes)”).

Note: This spec focuses on high availability of network resources. This spec does NOT address the scalability issue of process communications related to cell discussion nor underlying network topology related to network segment discussion.

Proposed Change

This change introduces the concept of an “Availability Zone” into Neutron. In particular, an availability zone is an optional attribute for Network and Router resources. These attributes in no way affect the behavior of Neutron in terms of allowed logical network connectivity. These attributes are simply used as hints to the backend about the location of other resources (compute and storage) that will be using these network resources. The Neutron backend may be able to use this to optimize its dynamic placement of resources to improve performance and/or ensure resources are placed in the same defined failure domain.

Create a new extension called availability_zone.

  • The extension adds a new API that lists availability zones.

The rest of this information applies to the implementation of availability zones for the built-in reference backend.

The extension adds the availability_zone attribute to Agent DB models. It also adds availability_zones and availability_zone_hints arrtibute for Network and Router DB models. The corresponding API resources will see the availability_zone attribute, too.

The new config options availability_zone and default_availability_zones are added. Availability zone of each agent is set by the availability_zone config parameter in each configuration file. If availability_zone parameter is not given in agent config, the agent is assigned to the default availability zone named “nova”. The name “nova” is referred to availability zone of Nova and Cinder. When a user executes resource create API without availability zone attribute, neutron set default_availability_zones value to the resource. The default_availability_zones value can be blank. If that’s the case, the scheduler selects any agent from any availability zone without any preference of specific availability zone. This helps to avoid the unbalance of resource assignment.

API and config are arranged to the following.

  • Using config, deployer specifies which availability_zone an agent belongs to, and they can also define default availability zones for user resources.
  • Using GET API of availability zone, users can get all the availability zones which neutron manages. API of availability zone is “GET” only.
  • Using GET API of network resources, users can get which availability zones their network resources is assigned.
  • Using POST API of network resources, users can create a network resource with availability zone hints as candidate for availability zone which the resource belongs to.

This spec enables each resource to belong to multiple availability zones. A user is able to specify the list of multiple AZs as a parameter when a resource is created. The list of multiple AZs defines the candidates of availability zone where the resource may be deployed. If the parameter at the creation is not given and the default_availability_zones config is not specified, the resource can be deployed at any availability zone. In other words, the list of multiple AZs for a resource restricts the scope of the deployment. Therefore, we can get redundancy by scheduling a network or a router to two agents in two distinct availability zones. Scheduler is also improved so that routers and networks are properly allocated with availability zone.

Limitations: With the reference L3 implementation without HA, we apparently cannot assign a router to multiple L3 agents and as a result we cannot achieve pure high availability from availability zone. A user just has an expectation of failure domain by setting availability zone to non-HA router. With L3-HA enabled router in the reference L3 implementation, all L3 agents across availability zones still need to have the connectivity to an external network uniformly to achieve high availability deployment.

Future work: It is definitely expected that all other services in neutron such as lbaas, fwaas, vpnaas and so on are able to handle the availability zone as its attributes. As these haven’t supported HA capability in the reference implementation yet, I suggest to implement them separately in another spec by step-by-step approach, hopefully almost concurrently with this spec.

Data Model Impact

As noted above, the spec adds availability_zone attribute to DB. A migration script will be provided. When operators update config, neutron checks different availability zone between resources and agents, then outputs some logs.

Attribute will be added:

Availability_zone attribute to RouterExtraAttributes

Attribute Type Description
availability_zone_hints String availability zone candidate for the router
availability_zones String availability zone for the router

Availability_zone attribute to NETWORKS as extend

Attribute Type Description
availability_zone_hints String availability zone candidate for the network
availability_zones String availability zone for the network

Availability_zone attribute to Agent

Attribute Type Description
availability_zone String availability zone for the agent


  • /agents

‘availability_zone’ key is added to ‘configurations’ attribute(dict). Note that ‘configurations’ attribute is read only.

  • /networks and /routers

The following attribute is added.

New attribute
Attribute Name Type Access Default Value Validation Conversion Description
availability_zone_hints list of string RW(POST only), all [] list of string list of human-readable name
availability_zones list of string RO, all [] list of string list of human-readable name
  • /availability_zones

The extension introduces a new availability_zone API resource. Only GET is available.

Attribute Name Type Access Default Value Validation Conversion Description
availability_zones list of dict RO, all N/A N/A see example below

An example of a JSON response:

    "availability_zones": [
            "name": "nova",
            "state": "available"

Security Impact


Notifications Impact


Other End User Impact

python-neutronclient and horizon will support new availability_zone value.

Performance Impact


IPv6 Impact

None. This proposal is protocol agnostic.

Other Deployer Impact

To make use of this feature, deployers need to set availability_zone in the each configuration file(e.g. l3_agent.ini and dhcp_agent.ini), specifying each network node’s availability zone.

The spec expects deployer to set an availability zone to an agent by config file since availability zone is related to a place of power socket and fixed equipment. However it doesn’t block new feature connected with availability zone from providing API, which enables deployer to specify availability zone without the config. It includes feature managing physical resources like Host_aggregation, Cell and others.

Upgrade Impact

Agent side: Before the upgrade, all agents are considered to be in the default availability zone named “nova.” Once an operator configures availability zone config parameter ‘availability_zone‘ in its agent config file and the agent is restarted, the agent belongs to the availability zone set in the config file. If an operator sets “nova” to the parameter, it means same as the default availability zone.

Resource side: Before the upgrade, all resources are considered to be at any availability zone. Even though an operator changes the availability zone of agents, it doesn’t break the matching to existing resources on the agent.

Developer Impact


Community Impact






Primary assignee:
Hirofumi Ichihara <ichihara-hirofumi>
Secondary assignee:
Iwamoto Toshihiro <iwamoto>

Work Items

  • Add availability_zone to the DB models
  • Make agents report their availability_zone settings
  • Add the availability_zone extension
  • (Validate REST API availability_zone parameters)
  • Add AvailabilityZoneFilter based on existing neutron scheduler implementations
  • Modify the L3(non-DVR and dvr_snat router) and DHCP agent schedulers to be AZ aware
  • Modify the L3(HA router) agent schedulers to be AZ aware
  • Add availability zone to python-neutronclient(Volunteers needed)
  • Add availability zone to horizon(assignee: amotoki)




Tempest Tests


Functional Tests

Add tests, which ensure resources are allocated for proper availability zone. Two new tests will be added for the following resources:

  • Network availability zone
  • Router availability zone

API Tests

Tests for the new attribute and the new API resource will be added.

Documentation Impact

User Documentation

The new config options will be documented. Availability zone use cases and the usage will be documented in the devref.

Developer Documentation