[RFE] Enable change of CIDR on a subnet

[Pedro Principeza]

[Request]
Reporting this RFE on behalf of a customer, who would like to inquire on the possibility of changing the CIDR of a subnet in Neutron.

As of today, the only alternative for expansion is to create new subnets/gateways to accomodate more hosts. The customer's desire is to avoid adding subnets and, instead, to change the existing one.

[Env]
This is a Bionic/Stein Juju-based deployment, but would apply to any supported OpenStack version.

[Workaround]
Create more subnets.

[Bence Romsics (bence-romsics)]

Hi Pedro,

The usual next step in discussing an RFE is to bring it to the neutron-drivers meeting:

http://eavesdrop.openstack.org/#Neutron_drivers_Meeting

Feel free to add this RFE to the agenda when you can attend the meeting:

https://wiki.openstack.org/wiki/Meetings/NeutronDrivers

I also recommend preparing some pros and cons why (for example here in this ticket) the suggested change is desirable.

Cheers,
Bence

[Slawek Kaplonski (slaweq)]

It sounds like pretty complex operation on the network/subnet with existing ports.
Can You elaborate more about Your use case and why it can't be solved with adding new subnets to the network?

[Pedro Principeza (pprincipeza)]

The idea here is to avoid having more routes and gateways, something that comes in when adding new subnets. Even if this causes changes for the Instances using the subnet (i.e. having to reconfigure IPs / Netmasks, or inherit that from the DHCP - if used), this would come handy, sysadmin-wise.

[Slawek Kaplonski (slaweq)]

I would like to discuss it on next Neutron drivers meeting which will be on Friday: http://eavesdrop.openstack.org/#Neutron_drivers_Meeting - so it would be great if You could join there if there would be any additional questions. But RFE should be discussed even if You will not be able to attend this meeting.

[Slawek Kaplonski (slaweq)]

During our drivers meeting we agreed to approve this rfe. One "limitation" which we would like to see here is that it should only allow to expand exisiting cidr of the subnet (like e.g. from /26 to /24), not changing it to something totally different.

Please now propose spec so we can review it and discuss details of this proposal and e.g. API changes. Then we can go to the implementation of this rfe.

[Slawek Kaplonski (slaweq)]

I registered BP https://blueprints.launchpad.net/neutron/+spec/extend-subnet-cidr to track this RFE.

[Pedro Principeza (pprincipeza)]

Thanks for that, Slawek. I'll submit the SPEC as soon as possible.

[Rodolfo Alonso (rodolfo-alonso-hernandez)]

RFE implementation not attended, reopen if needed.