In this talk, we will describe B4, the SDN-based network that connects Google’s data centers to each other, and some of the lessons we have learned over several years of operational experience. Background: B4 is one of Google’s two backbone networks. Originally designed as a low cost backend backbone, specifically for carrying high volume copy traffic, B4 has grown into a critical infrastructure for Google. Currently it carries more traffic than Google’s public facing backbone. B4 is built with a home-grown software stack and home-grown switch hardware, based on merchant silicon ASICs.