DVR is supposed to address following problems which has traditional 3 Node
Problem 1: Intra VM traffic flows through the Network Node
In this case even VMs traffic that belong to the same tenant
on a different subnet has to hit the Network Node to get routed
between the subnets. This would affect Performance.
Problem 2: VMs with FloatingIP also receive and send packets
through the Network Node Routers.
FloatingIP (DNAT) translation done at the Network Node and also
the external network gateway port is available only at the Network Node.
So any traffic that is intended for the External Network from
the VM will have to go through the Network Node.
In this case the Network Node becomes a single point of failure
and also the traffic load will be heavy in the Network Node.
This would affect the performance and scalability.
Complete text of post may be seen here