Support multiple interfaces from one VM attached to the same network

Permit VMs to attach multiple interfaces to one network to facilitate use of common NFV network function VMs that require this form of attachment.

Problem description

At present, Nova only permits a single VIF from each VM to be attached to a given Neutron network. If you attempt to attach multiple VIFs to the same network, an error is issued, meaning that the second network is not found from the list of networks remaining after the first network is not used.

NFV functions occasionally require multiple interfaces to be attached to a single network from the same VM, for reasons described below in the ‘use cases’ section. When this is required, the VNF generally cannot be used under Openstack.

VNFs are often large, complex pieces of code, and may be supplied by third parties. For various reasons, it is not uncommon that it is necessary to feed traffic out of an interface and into another interface (when the VNF implements multiple functions and the functions cannot be chained internally) or to feed traffic from e.g. the internet into multiple interfaces to run them through separate processing functions internally.

The limitation can be seen as one of the VNF. Clearly, the VNF could be changed to put multiple addresses or functions on a single port (to fix the incoming traffic issue) or to connect functions internally (to fix the passthrough problem.

The problem with this solution is that the timescale for getting such a fix is often prohibitive. VNFs are large, complex pieces of code, and often the supplier of the VNF is not the same organisation as that trying to use the VNF within Openstack, necessitating a feature change request which may well not be possible within reasonable timescales.

We propose changing the code within Nova to remove this limitation.

Proposed change

We propose removing the limitation, which exists in Nova (Neutron has no such limitation), allowing any number of VIFs to be attached to the same network.

The ordering in the nova ‘boot’ command or POST should be respected, so if multiple interfaces are in use on the same network they are attached to the VM in the order in which they are provided, as with other NICs.

API changes

When the attempt is made to attach multiple interfaces to a single network, Nova will, instead of returning the error, attach multiple interfaces to the same network and return a normal success code to the ‘nova boot’ attempt.

(‘nova interface-attach’ already permits a second attachment to the same network and needs no change.)


It may be possible to work around this limitation by using multiple ports on the same network and attach the VM to the ports, rather than the same network twice. This has not been tested. On the other hand, this indicates that the limitation is highly artificial and should, in any case, be removed. (In any case we should confirm this is possible after the change and fix it if not.)

It is possible to boot the VM and use ‘nova interface-attach’ to get multiple interfaces on the same network, but this requires the VM to support PCI hotplug.

Data model impact


REST API impact

When the attempt is made to attach multiple interfaces to a single network, Nova will, instead of returning the error, attach all interfaces to the same network and return a normal success code to the ‘nova boot’ attempt.

Security impact

It is now going to be possible to bridge multiple interfaces together within a VM and cause a broadcast storm. It was always possible to flood a Neutron network from a VM; this makes it easier. It doesn’t make a security issue in and of itself but it certainly does make it a little more straightforward to trigger one that arguably already exists.

Notifications impact


Other end user impact

None. An end user using API calls that currently succeed will see no change in behaviour in those APIs. This only changes a case where an API currently fails.

Performance Impact


Other deployer impact


Developer impact




Primary assignee:


Work Items

  • Change the Nova code, per the existing abandoned patch in - which requires porting forward from the code in question to the current trunk.

  • Add unit tests, which are missing from the abandoned patch.




Independently of this spec, tests should be added to Tempest:

  • minimally, to ensure that traffic can be passed between the two interfaces on a VM created in this fashion

  • optionally, traffic flow should be tested from another VM or external packet supplier to either of the interfaces.

Testing should be conducted with both the nova boot and nova interface-attach methods.

Documentation Impact

The change should be documented. No documentation exists for the current behaviour. Documentation exists for nova-network multinic saying that VIFs are attached to separate networks but this is specific to nova-network.