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You will have to target two IP addresses with DHCP relay if you are
using Ironic Inspector. The first is the IP where Ironic Inspector is
listening with dnsmasq, usually the IP of the host itself. I know this
doesn't lend itself to HA scenarios, but you might also be able to
forward to the broadcast IP of the subnet where the Ironic Inspector
will be running (I haven't tested this, but it is a common use case for
DHCP relay).

The second IP address is that of the Neutron DHCP agent, and that will
be used for deploying bare metal nodes. IIRC, this IP is shared with
the Neutron router for the network if you are using the L3 agent as
well.

If you are not running Ironic Inspector (and manually entering in
baremetal host details instead), then you can forward DHCP relay only
to the Neutron DHCP agent.

Both of these IP addresses will be on the "root" subnet which is
associated with the segment with the controller node(s).

It sounds like you created a second subnet, but I'm not sure if you
created the second subnet on a different segment from the first subnet.
In Neutron routed networking, the segments determine whether a subnet
is local or remote to the controller node(s). Typically the first
segment would be the one local to the controller(s). Are you sure you
enabled the segments plugin and created your second subnet on a new
segment?

Another approach which does not involve DHCP relay is to deploy DHCP
agents locally on compute nodes local to each segment. This way all
DHCP will be done within the same L2 domain, and you will not have to
configure DHCP relay on your router serving each segment/subnet.

See the docs for more info:
https://docs.openstack.org/newton/networking-guide/config-routed-networks.html

-Dan

On Fri, 2020-05-29 at 10:47 -0600, Thomas King wrote:
> In the Triple-O docs for unicast DHCP relay, it doesn't exactly say
> which IP address to target. Without deploying Triple-O, I'm not clear
> if the relay IP should be the bridge interface or the DHCP device. 
> 
> The first method makes sense because the gateway for that subnet
> wouldn't be connected to the Ironic controller by layer 2 (unless we
> used VXLAN over the physical network). 
> 
> As an experiment, I created a second subnet on my provisioning
> network. The original DHCP device port now has two IP addresses, one
> on each subnet. That makes the second method possible if I targeted
> its original IP address. 
> 
> Thanks for the help and please let me know which method is correct. 
> 
> Tom King
> 
> On Fri, May 29, 2020 at 3:15 AM Dan Sneddon <dsneddon at redhat.com>
> wrote:
> > You probably want to enable Neutron segments and use the Neutron
> > routed networks feature so you can use different subnets on
> > different segments (layer 2 domains AKA VLANs) of the same network.
> > You specify different values such as IP allocation pools and router
> > address(es) for each subnet, and Ironic and Neutron will do the
> > right thing. You need to enable segments in the Neutron
> > configuration and restart the Neutron server.  I don’t think you
> > will have to recreate the network. Behind the scenes, dnsmasq will
> > be configured with multiple subnets and address scopes within the
> > Neutron DHCP agent and the Ironic Inspector agent. 
> > 
> > Each segment/subnet will be given a different VLAN ID. As Dmitry
> > mentioned, TripleO uses that method for the provisioning network,
> > so you can use that as an example. The provisioning network in
> > TripleO is the one referred to as the “control plane” network. 
> > 
> > -Dan
> > 
> > On Fri, May 29, 2020 at 12:51 AM Dmitry Tantsur <
> > dtantsur at redhat.com> wrote:
> > > Hi Tom,
> > > 
> > > I know for sure that people are using DHCP relay with ironic, I
> > > think the TripleO documentation may give you some hints (adjusted
> > > to your presumably non-TripleO environment): 
> > > http://tripleo.org/install/advanced_deployment/routed_spine_leaf_network.html#dhcp-relay-configuration
> > > 
> > > Dmitry
> > > 
> > > On Thu, May 28, 2020 at 11:06 PM Amy Marrich <amy at demarco.com>
> > > wrote:
> > > > Hey Tom,
> > > > 
> > > > Forwarding to the OpenStack discuss list where you might get
> > > > more assistance.
> > > > 
> > > > Thanks,
> > > > 
> > > > Amy (spotz)
> > > > 
> > > > On Thu, May 28, 2020 at 3:32 PM Thomas King <
> > > > thomas.king at gmail.com> wrote:
> > > > > Good day, 
> > > > > 
> > > > > We have Ironic running and connected via VLANs to nearby
> > > > > machines. We want to extend this to other parts of our
> > > > > product development lab without extending VLANs. 
> > > > > 
> > > > > Using DHCP relay, we would point to a single IP address to
> > > > > serve DHCP requests but I'm not entirely sure of the Neutron
> > > > > network/subnet configuration, nor which IP address should be
> > > > > used for the relay agent on the switch. 
> > > > > 
> > > > > Is DHCP relay supported by Neutron? 
> > > > > 
> > > > > My guess is to add a subnet in the provisioning network and
> > > > > point the relay agent to the linuxbridge interface's IP: 
> > > > > 14: brq467f6775-be: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu
> > > > > 1500 qdisc noqueue state UP group default qlen 1000
> > > > >     link/ether e2:e9:09:7f:89:0b brd ff:ff:ff:ff:ff:ff
> > > > >     inet 10.10.0.1/16 scope global brq467f6775-be
> > > > >        valid_lft forever preferred_lft forever
> > > > >     inet6 fe80::5400:52ff:fe85:d33d/64 scope link 
> > > > >        valid_lft forever preferred_lft forever
> > > > > 
> > > > > Thank you, 
> > > > > Tom King
> > > > > _______________________________________________
> > > > > openstack-mentoring mailing list
> > > > > openstack-mentoring at lists.openstack.org
> > > > > http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-mentoring
> > 
> > -- 
> > Dan Sneddon         |  Senior Principal Software Engineer
> > dsneddon at redhat.com |  redhat.com/cloud
> > dsneddon:irc        |  @dxs:twitter
-- 
Dan Sneddon         |  Senior Principal Software Engineer
dsneddon at redhat.com |  redhat.com/cloud
dsneddon:irc        |  @dxs:twitter