[openstack-dev] [nova] Multiple default routes in instance

Mathieu Gagné mgagne at calavera.ca
Wed May 17 14:08:17 UTC 2017

On Wed, May 17, 2017 at 10:03 AM, Mathieu Gagné <mgagne at calavera.ca> wrote:
> Hi,
> When you attach multiple networks/interfaces to an instance and at
> least 2 subnets have a default route, you end up with 2 default routes
> in network_data.json.
> If cloud-init or any other in-guest agent is used to configure the
> network, you could end up with 2 default routes and to an extend, with
> non-working networking. (like we found out recently)
> How should we handle this situation where you could end up with 2
> subnets, each having a default route?
> * Should Nova decide which default route to expose to the instance?
> * Should the in-guest agent pick one?
> I'm looking for opinions and advises on how to fix this issue.

I would also like to draw attention to a spec I wrote which faces the same
questions and challenges related to multiple fixed-ips per port support:

Quoting the commit message here:

> Each subnet can have its own routes and default route which create
> * Should all routes be merged in the same "routes" attribute?
> * What should be the default route if more than one subnet provides a
default route? Should we provide all of them?
> * What should be the implementation logic an in-guest agent use to
determine the default route?
> * Are there use cases where one default route should be used over the
others? I'm thinking about NFV or other specialized use cases.

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