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@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@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@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@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
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