Scheduling support for Routed Networks

Neutron provides network segments support thanks to Routed Networks where you can create a port allocated to a specific segment. Unfortunately, Nova doesn’t verify the segment at every instance operation, including those where you move an instance, which leads to inconsistencies.

Problem description

Although it’s possible to create a Neutron port with a routed networks setup and boot an instance with this port, the network locality of the compute node associated with the instance won’t be verified by the scheduler and could lead to a wrong scheduling decision. This is problematic when a move operation sends an instance to a compute node that isn’t in the network segment that is related to the IP address that was allocated at boot time.

As a result of this gap, the only way to use routed networks in Nova currently is by creating a port having the ip_allocation value be deferred and making sure that all compute services are assigned to at least one network segment.

Use Cases

As an operator, I’d like to make sure that instances IP addresses can be correctly separated between the network segments I provided.

As an operator, I don’t want to see instances going to compute services that aren’t in network segments if the user asks for either a port or a routed network.

As a user, I’d like Nova to place my instance on the correct host according to the port or network I’ve requested for my instance, without having to specifically create a port with a ip_allocation=deferred value.

Proposed change

Once you configure routed networks in Neutron, network segments are represented as Placement Resource Providers. Neutron will then ask Nova to create a Nova host aggregate for each segment and will add compute services that are mapped with respective segments into the related aggregates. Eventually, Nova will mirror those aggregates into Placement aggregates.

What we then need for Nova is to have a way for asking the Placement API to only get resource providers (i.e. compute nodes) that are in the aggregate related to the segments that are in the network passed by the user (or related to the port that is asked).

As Nova needs to find which segments are related and then which aggregates, we could just provide a new pre-filter that would look at it if some configuration option (say query_placement_for_routed_network_aggregates) would be True.

A pseudo-code for it would be :

def support_routed_networks(ctxt, request_spec):
  if not CONF.query_placement_for_routed_network_aggregates:
    return False
  segment_ids = <get_all_segments_ids_from_network_or_port>
  for segment in segment_ids:
     agg_info = <get_provider_aggregates_from_segment_id>


As said below in the Alternatives section, we could have Neutron passing directly the aggregates, so this pre-filter could be deprecated once we do it.


Instead of having a new pre-filter, we could provide a specific scheduler filter. This said, given we limit the number of allocation candidates returned by Placement, we could miss some good resource providers so the filter couldn’t work.

Another alternative would be to have Neutron passing directly the needed aggregate to Nova instead of Nova asking Neutron for it, but that would mean that we should modify Neutron to return the Placement needed query in the port.resource_request attribute.

Data model impact

We may need to augment the RequestSpec object to be able to provide in its nested RequestLevelParams object attribute the specific aggregate.

REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact

There could be a performance impact if we would verify the segments for every instance in every cloud, but given we ask the operator to modify an option if they want to use routed networks, we don’t really think this would be an issue.

Other deployer impact

A new configuration option would be:


Developer impact


Upgrade impact




Primary assignee:


Other contributors:


Feature Liaison


Work Items

  • Create a new pre-filter that would find the related aggregate

  • Pass the aggregate to the RequestSpec asking to verify it by Placement

  • That’s it.




Functional tests of course, but Tempest tests would be nice as well.

Documentation Impact

Maybe modifying





Release Name



Introduced, Approved