The auto-allocation feature introduced in Mitaka simplifies the
procedure of setting up an external connectivity for end-users, and is
also known as Get Me A Network.

Previously, a user had to configure a range of networking resources
to boot a server and get access to the Internet. For example, the
following steps are required:

• Create a network
• Create a subnet
• Create a router
• Uplink the router on an external network
• Downlink the router on the previously created subnet

These steps need to be performed on each logical segment that a VM
needs to be connected to, and may require networking knowledge the user
might not have.

This feature is designed to automate the basic networking
provisioning for projects. The steps to provision a basic network are
run during instance boot, making the networking setup hands-free.

To make this possible, provide a default external network and default
subnetpools (one for IPv4, or one for IPv6, or one of each) so that the
Networking service can choose what to do in lieu of input. Once these
are in place, users can boot their VMs without specifying any networking
details. The Compute service will then use this feature automatically to
wire user VMs.

Enabling the
deployment for auto-allocation

To use this feature, the neutron service must have the following
extensions enabled:

• auto-allocated-topology
• subnet_allocation
• external-net
• router

Before the end-user can use the auto-allocation feature, the operator
must create the resources that will be used for the auto-allocated
network topology creation. To perform this task, proceed with the
following steps:

1. Set up a default external network

   Setting up an external network is described in OpenStack Networking Guide.
   Assuming the external network to be used for the auto-allocation feature
   is named public, make it the default external
   network with the following command:

   $ openstack network set public --default

   Note

   The flag --default (and --no-default flag)
   is only effective with external networks and has no effects on regular
   (or internal) networks.

2. Create default subnetpools

   The auto-allocation feature requires at least one default subnetpool.
   One for IPv4, or one for IPv6, or one of each.

   $ openstack subnet pool create --share --default \
     --pool-prefix 192.0.2.0/24 --default-prefix-length 26 \
     shared-default
   
     +-------------------+--------------------------------------+
     | Field             | Value                                |
     +-------------------+--------------------------------------+
     | address_scope_id  | None                                 |
     | created_at        | 2017-01-12T15:10:34Z                 |
     | default_prefixlen | 26                                   |
     | default_quota     | None                                 |
     | description       |                                      |
     | headers           |                                      |
     | id                | b41b7b9c-de57-4c19-b1c5-731985bceb7f |
     | ip_version        | 4                                    |
     | is_default        | True                                 |
     | max_prefixlen     | 32                                   |
     | min_prefixlen     | 8                                    |
     | name              | shared-default                       |
     | prefixes          | 192.0.2.0/24                         |
     | project_id        | 86acdbd1d72745fd8e8320edd7543400     |
     | revision_number   | 1                                    |
     | shared            | True                                 |
     | tags              | []                                   |
     | updated_at        | 2017-01-12T15:10:34Z                 |
     +-------------------+--------------------------------------+
   
   $ openstack subnet pool create --share --default \
     --pool-prefix 2001:db8:8000::/48 --default-prefix-length 64 \
     default-v6
   
   +-------------------+--------------------------------------+
   | Field             | Value                                |
   +-------------------+--------------------------------------+
   | address_scope_id  | None                                 |
   | created_at        | 2017-01-12T15:14:35Z                 |
   | default_prefixlen | 64                                   |
   | default_quota     | None                                 |
   | description       |                                      |
   | headers           |                                      |
   | id                | 6f387016-17f0-4564-96ad-e34775b6ea14 |
   | ip_version        | 6                                    |
   | is_default        | True                                 |
   | max_prefixlen     | 128                                  |
   | min_prefixlen     | 64                                   |
   | name              | default-v6                           |
   | prefixes          | 2001:db8:8000::/48                   |
   | project_id        | 86acdbd1d72745fd8e8320edd7543400     |
   | revision_number   | 1                                    |
   | shared            | True                                 |
   | tags              | []                                   |
   | updated_at        | 2017-01-12T15:14:35Z                 |
   +-------------------+--------------------------------------+

Get Me A Network

In a deployment where the operator has set up the resources as
described above, they can get their auto-allocated network topology as
follows:

$ openstack network auto allocated topology create --or-show
+------------+--------------------------------------+
| Field      | Value                                |
+------------+--------------------------------------+
| id         | a380c780-d6cd-4510-a4c0-1a6ec9b85a29 |
| name       | None                                 |
| project_id | cfd1889ac7d64ad891d4f20aef9f8d7c     |
+------------+--------------------------------------+

Operators (and users with admin role) can get the auto-allocated
topology for a project by specifying the project ID:

$ openstack network auto allocated topology create --project \
  cfd1889ac7d64ad891d4f20aef9f8d7c --or-show
+------------+--------------------------------------+
| Field      | Value                                |
+------------+--------------------------------------+
| id         | a380c780-d6cd-4510-a4c0-1a6ec9b85a29 |
| name       | None                                 |
| project_id | cfd1889ac7d64ad891d4f20aef9f8d7c     |
+------------+--------------------------------------+

The ID returned by this command is a network which can be used for
booting a VM.

$ openstack server create --flavor m1.small --image \
  cirros-0.3.5-x86_64-uec --nic \
  net-id=8b835bfb-cae2-4acc-b53f-c16bb5f9a7d0 vm1

The auto-allocated topology for a user never changes. In practice,
when a user boots a server omitting the --nic option, and
there is more than one network available, the Compute service will
invoke the API behind auto allocated topology create, fetch
the network UUID, and pass it on during the boot process.

Validating the
requirements for auto-allocation

To validate that the required resources are correctly set up for
auto-allocation, without actually provisioning anything, use the
--check-resources option:

$ openstack network auto allocated topology create --check-resources
Deployment error: No default router:external network.

$ openstack network set public --default

$ openstack network auto allocated topology create --check-resources
Deployment error: No default subnetpools defined.

$ openstack subnet pool set shared-default --default

$ openstack network auto allocated topology create --check-resources
+---------+-------+
| Field   | Value |
+---------+-------+
| dry-run | pass  |
+---------+-------+

The validation option behaves identically for all users. However, it
is considered primarily an admin or service utility since it is the
operator who must set up the requirements.

Project resources
created by auto-allocation

The auto-allocation feature creates one network topology in every
project where it is used. The auto-allocated network topology for a
project contains the following resources:

Compatibility notes

Nova uses the auto allocated topology feature with API
micro version 2.37 or later. This is because, unlike the neutron feature
which was implemented in the Mitaka release, the integration for nova
was completed during the Newton release cycle. Note that the CLI option
--nic can be omitted regardless of the microversion used as
long as there is no more than one network available to the project, in
which case nova fails with a 400 error because it does not know which
network to use. Furthermore, nova does not start using the feature,
regardless of whether or not a user requests micro version 2.37 or
later, unless all of the nova-compute services are running
Newton-level code.