OpenContrail is a SDN solution which offers additional networking features such as vDNS, IPAM and security ACL on both subnets and individual VM levels. It has a centralized controller serving as control plane and a distributed virtual router serving as forwarding/data plane. OpenContrail vrouter operates on each hypervisor abd VM is attached to the vRouter by VRF (virtual Forward Routing). The forwarding plane supports MPLS over GRE, VXLAN, L2/3 unicase and L3 Multicase for interconnection between virtual and physical networks. This http://www.opencontrail.org/opencontrail-architecture-documentation/#section1 (page) has excellent architecture overview for OpenContrail.

Basically the Contrail is to replace the Neutron in Openstack. Contrail is highly scalabe where the Openstack-Neutron will panic. Following video provides a glance of networking scalability using Contrail. Have a look.

https://www.youtube.com/watch?v=xN0rXHD_dqk (https://www.youtube.com/watch?v=xN0rX...)

Scalability, Reliability, Performance, Fully Distributed Networking - Switching, Routing, NAT, DHCP, DNS, etc.