[openstack-dev] [neutron][networking-ovn] Integration with OVN NAT gateway (Proposal)

Amitabha Biswas azbiswas at gmail.com
Tue Jun 7 20:53:24 UTC 2016

This proposal outlines the modifications needed in networking-ovn (addresses https://bugs.launchpad.net/networking-ovn/+bug/1551717 <https://bugs.launchpad.net/networking-ovn/+bug/1551717>) to provide Floating IP (FIP) and SNAT using the L3 gateway router patches.

http://patchwork.ozlabs.org/patch/624312/ <http://patchwork.ozlabs.org/patch/624312/> 
http://patchwork.ozlabs.org/patch/624313/ <http://patchwork.ozlabs.org/patch/624312/> 
http://patchwork.ozlabs.org/patch/624314/ <http://patchwork.ozlabs.org/patch/624312/> 
http://patchwork.ozlabs.org/patch/624315/ <http://patchwork.ozlabs.org/patch/624312/> 
http://patchwork.ozlabs.org/patch/629607/ <http://patchwork.ozlabs.org/patch/629607/>


+-------+                       +-------+
| NET 1 |                       | NET 2 |
+-------+                       +-------+
   |                               |
   |           *********           |
   |         **         **         |
   |       **    *  *     **       |
   +---RP1 *      DR       * RP2 --+
           **    *  *     **
             **         **  
                  DTRP (
           | Transit Network  |
           | |
                  GTRP (
              **       **   
            **   *   *   **                 +------------------+
            *     GW      *-----------------| Provider Network |
            **   *   *   **                 +------------------+
              **       **   

New Entities:

OVN Join/Transit Networks
One per Neutron Router - /30 address space with only 2 ports for e.g.
Created when an external gateway is added to a router.
One extra datapath per router with an External Gateway.
(Alternate option - One Transit Network in a deployment, IPAM becomes a headache - Not discussed here).
Prevent Neutron from using that /30 address space. Specify in networking-ovn conf file.
Create 1 new “Join” neutron network (to represent all Join OVN Networks) in the networking-ovn.
Note that it may be possible to replace the Join/Transit network using Router Peering in later versions  (not discussed here).
Allocate 2 ports in the Join network in the networking-ovn plugin.
Logical Gateway Transit Router Port (gtrp),
Logical Distributed Transit Router Port (dtrp),
Note that Neutron only sees 1 Join network with 2 ports; OVN sees a replica of this Join network as a new Logical Switch for each Gateway Router. The mapping of OVN Logical Switch(es) Join(s) to Gateway Router is discussed in OVN (Default) Gateway Routers below.
Note that the MAC addresses of lgrp and lip will be the same on each OVN Join Network, but because they are in different branches of the network topology it doesn’t matter.
OVN (Default) Gateway Routers:
One per Neutron Router.
2 ports
Logical Gateway Transit Router Port (gtrp), (same for each OVN Join network).
External/Provider Router Port (legwrp), this is allocated by neutron.
Scheduling - The current OVN gateway proposal relies on the CMS/nbctl to decide on which hypervisor (HV) to schedule a particular gateway router.
A setting on the chassis (new external_id key or a new column) that allows the hypervisor admin to specify that a chassis can or cannot be used to host a gateway router (similar to a network node in OpenStack). Default - Allow (for compatibility purposes).
The networking-ovn plugin picks up the list of “candidate” chassis from the Southbound DB and uses an existing scheduling algorithm
Use a simple random.choice i.e. ChanceScheduler (Version 1)
Tap into the neutron’s LeastRouterScheduler - but that requires the networking-ovn (or some a hacked up version of the L3 agent) to imitate the L3 agent running on various network nodes.
Populate the SNAT and DNAT columns in the logical router table. This is under review in OVS - http://openvswitch.org/pipermail/dev/2016-June/072169.html <http://openvswitch.org/pipermail/dev/2016-June/072169.html>
Create static routing entry in the gateway router to route tenant bound traffic to the distributed logical router.ar gate

Existing Entities:

Distributed Logical Routers:
Set the default gateway of the distributed logical router to the IP Address of the corresponding Logical Gateway Transit Router Port (

It would be good to get some feedback on this strategy. Guru mentioned that he saw a need for ARP response across multiple gateway routers, we don’t see that requirement in this design/use-case.

Amitabha (azbiswas) and Chandra (chandrav)

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