[openstack-dev] [qa] Should it be allowed to attach 2 interfaces from the same subnet to a VM?

Steve Gordon sgordon at redhat.com
Tue Dec 2 16:59:21 UTC 2014

----- Original Message -----
> From: "Danny Choi (dannchoi)" <dannchoi at cisco.com>
> To: openstack-dev at lists.openstack.org
> Hi Andrea,
> Though both interfaces come up, only one will response to the ping from the
> neutron router.
> When I disable it, then the second one will response to ping.
> So it looks like only one interface is useful at a time.
> My question is is there any useful case for this, I.e. Why would you do this?
> Thanks,
> Danny

The rationale is given in the spec (http://specs.openstack.org/openstack/nova-specs/specs/juno/implemented/nfv-multiple-if-1-net.html) as:

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.


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