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1b1ed1a
(Get the code!)
I think there are a few misconceptions. Hopefully Ryan's diagrams will help but I'll try to set a few things straight.
This wouldn't be replacing the software router with the provider router for N/S. Both routers are in the N/S datapath in all cases. However, you're right in thinking that we are looping in the provider router and taking advantage of its capabilities.
Address scopes give us the capability to take NAT out of the picture when routing N/S. My recent contribution to the networking guide [1] (which merged!) explains how that works.
Consider this with BGP dynamic routing. It gives us the option to peer Neutron with the provider router. We could send host routes to it so that it will send southbound traffic directly to the DVR router on the correct compute host with the internal port. We left this out of scope for Mitaka (except when floating IPs are used) for two reasons. 1) The fip namespace was not constructed properly when we needed it because it was only constructed on demand to service floating ips. 2) The northbound traffic would still get redirected to the network node creating asymmetric routing paths for northbound and southbound.
This RFE is about allowing a fast exit datapath for northbound traffic that could match the host route path for southbound traffic and keep the paths symmetric. In my view, this requires some knowledge on Neutron's part of the routing provider (BGP) that is setting up the host routes.
[1] https:/