I would like to understand, what is the exact role of the node level bridge (kbr0/docker0)in case of Kubernetes overlay networking presented here: https://github.com/GoogleCloudPlatform/kubernetes/blob/master/docs/ovs-networking.md https://github.com/coreos/flannel#flannel

I understand how overlay networks work, in general. However, it is not clear for me why we need that node specific bridge? Is it only to provide a bridge for docker? Cannot OvS or Flannel directly handle the interfaces to the containers? If I consider the analogy of VMs in a cloud and distributed switches, there is no node/host specific bridge between the distributed switch and the VM.