Supports communication between different subnets within the same network

[howon park]

There are multiple subnets in one network.
Each subnet is connected to a different router and communicates with its own path.

Additional router and static routing settings are required to communicate between subnets.

This feels unreasonable to me, who are accustomed to the aws vpc environment.

Is there no way because it doesn't fit the network purpose of openstack?
Or maybe I didn't find another simple way to do it

Please give me feedback.

Thank you...!

[Miro Tomaska (mtomaska)]

Hi Howon,

Are you suggesting that theree should be some automatic way for all subnets being reachable simply because they are part of the same network?

What would be a real world use case for such feature? Besides the obvious, convenience and less steps to bring up such network.

Right now, if you create a network with multiple subnets and add a router between them then those subnets would be reachable. However, if a subnet is already connected to some other router you will have to create a new router port on that subnet which you can then connect to your new router.

[howon park (howonpark)]

Hi Miro Tomaska,

In an ovn network environment, subnet is separated by dhcp and has been confirmed to belong to the same one broadcast domain.

I can branch paths through router isolation, but I haven't found a way to divide the network broadcast area.
If full broadcast isolation is just a way to create a new network, it feels strange that the only way to communicate between subnets in one network is through router connections.
I don't think this method is much different from connecting other networks.

For this reason, I think we need to connect the same network L2 area other than L3 connection through the router.
Or even if you use a router, I hope there is a way other than adding a separate port or router.

Thank you for your response.
Regards.

[Brian Haley (brian-haley)]

Hi Howon, let me see if I understand your question.

With a network that has a number of subnets, you can typically install static interface routes to each one and communicate directly with VMs on those subnets, instead of forwarding via the router. With ML2/OVS the dhcp-agent does actually do this and it works.

With OVN I'm not sure the built-in DHCP responder does this, but since everything is just switched is there really a need to?

Is the goal just to eliminate the router "hop" ?

[howon park (howonpark)]

Hi Brian Haley,

Is it possible for dhcp agents to communicate multiple subnets without going through the router in an ML2/ovs environment?

No such features were found in the OVN environment.
It exists, but maybe I didn't find it, but I couldn't find a way to configure it in an openstack environment.

The goal is not to remove the router's hops.
It would be nice if it could be removed, but even if it wasn't, I didn't want to increase the number of routers.

I need a way to connect for local communication without adding a router within the same ovn switch.
This increases the value of the subnet.

Currently, the benefits of having multiple subnets in a network are unknown. It is not possible to separate the network into one broadcast area, but it is a pity that communication is not possible.
Creating a new network and using only one subnet each seems more appropriate for your current environment.

It could just be my one opinion.
Thank you for your response.
Regards.

[howon park (howonpark)]

I think the reason for using multiple subnets in the network is to branch the routing path.

However, there is no alternative to maintaining the connection of subnets within the same network at routing branching

This does not seem to have an advantage in using the same network.

It seems appropriate to use one subnet for your network. If you're going to connect with a router anyway...

It talks about the need for a way to do this.

So I found a way.
ovn Logical_Router_Static_Route 에
route_table allows you to branch static routing for each router port.
This requires only one router to connect between subnets.

Is there any technical analysis potential for this?

https://review.opendev.org/c/openstack/ovsdbapp/+/896793

Please review it.

[Brian Haley (brian-haley)]

> Is it possible for dhcp agents to communicate multiple subnets without going through the router in an ML2/ovs environment?

From my reading of the code I think it does this, see neutron/agent/linux/dhcp.py:_generate_opts_per_subnet(), but I could be wrong.

> I need a way to connect for local communication without adding a router within the same ovn switch.

Ok, so multiple subnets on the same network, but without a router?

> Is there any technical analysis potential for this?
>
> https://review.opendev.org/c/openstack/ovsdbapp/+/896793

Sorry, I don't know much about that patch, you could maybe ping Terry Wilson (otherwiseguy) on irc to see if he can add anything as I don't see neutron consuming it.

[howon park (howonpark)]

> From my reading of the code I think it does this, see neutron/agent/linux/dhcp.py:_generate_opts_per_subnet(), but I could be wrong.
I don't use ovs environment, but I'll keep that in mind. Thank you.
>Ok, so multiple subnets on the same network, but without a router?
Yes, I hope it's possible without a router.

However, I think this might seem strange in a typical network.

I thought about applying the concept of a subnet pool to the network so that divided subnets can communicate within the subnet pool.
Wouldn't it be clearer since it's a sub-prefix of the subnet pool?
It's just one opinion

> Sorry, I don't know much about that patch, you could maybe ping Terry Wilson (otherwiseguy) on irc to see if he can add anything as I don't see neutron consuming it.
I'm sorry. I confused you..
This is a patch that has registered the ovn routing function as a library.
I haven't seen any use cases yet with patches that have opened up availability.
I'll analyze it a bit more

Thank you