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Hi Carl.

Adding some numbers from CERN to the discussion, we currently have:
* ~40 cells
* ~185 segments (called clusters in the our doc linked in the etherpad)
* ~185*10 subnets, although it's not easy to compute the actual number
today in our nova-network setup

With the move to neutron (new cells are being deployed with it, the
existing capacity is to migrated in the next couple months) we're
doing some consolidation, and might aim at having number of cells ==
number of segments (or not far from it). The ratio of subnets should
be dropped significantly too.

Regarding the dhcp agent topic Belmiro described, the need to deploy a
dhcp agent in the first place came to overcome an issue integrating
with the CERN dhcp - not related to neutron, and mostly to speed up
the launch of new VMs. As the neutron dhcp agent does not work for our
setup, this is what we're looking at (and seems to work):
- run a dhcp daemon (our own) on every compute node, it talks to
neutron to build the replies
- block dhcp traffic accordingly so that requests are handled by our agent

Running a neutron agent per segment would be ideal, but where exactly
would that be? We're running one central neutron control plane
(network node(s)) which won't get the dhcp requests, and the only
alternative was to put it down in the compute nodes. Even if we would
move to a network node per cell this wouldn't be enough if we end up
with
multiple segments in a single cell.

Cheers,
  Ricardo

On Fri, Feb 26, 2016 at 7:47 PM, Carl Baldwin <carl at ecbaldwin.net> wrote:
> On Fri, Feb 26, 2016 at 4:21 AM, Belmiro Moreira
> <moreira.belmiro.email.lists at gmail.com> wrote:
>> Hi,
>> thanks Carl for info about the DHCP plans.
>
> Olá, Belmiro.
>
>> Our DHCP concern is because currently the DHCP agent needs to be assigned
>> to a network and then it creates a port for each subnet.
>
> Right, this is a concern for me too.
>
>> In our infrastructure we only consider a network with several hundred
>> subnets.
>> By default the DHCP agent runs in the network node however when using
>> provider
>> networks and segmentation the DHCP requests will not reach it.
>
> Yep, that is the issue.
>
>> My understanding is that the plan is to have the DHCP agent per segment.
>> That is great.
>
> Correct.
>
>> But it will continue to create a port per subnet?
>> Looking into our usecase (only provided networks with segmentation) I don't
>> see why it
>> should create ports.
>
> So, we'll need to look in to this.  It might be something we can look
> at independently.  First, I'd like a little more information on your
> "several hundred subnets."  Do you see these mapping to hundreds of
> segments?  I guess what I'm after is "what do you envision will be
> your ratio of subnets to segments?"
>
> Carl
>
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