Overlay Networks: Overlay networks create a virtual network on top of an existing physical network. This is achieved by encapsulating the original packet (the pod's network traffic) within another packet (the host's network traffic). This "tunneling" allows traffic to traverse different physical networks as if they were a single, unified network.
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A) The Container Network Interface (CNI) automatically creates a physical network bridge between the nodes' NICs: The CNI is responsible for configuring network interfaces for containers, but it doesn't create physical bridges between nodes' NICs. That would require physical rewiring or complex network configurations outside the scope of Kubernetes and the CNI.
C) Kubernetes' built-in service mesh handles all cross-node pod communication without needing an overlay network: While a service mesh (like Istio or Linkerd) enhances service-to-service communication with features like traffic management, security, and observability, it does not provide the fundamental network connectivity between pods on different nodes. An underlying network solution (like an overlay network or a routed pod network) is still required.
D) The kubelet service maintains a direct VPN tunnel between every pair of nodes in the cluster: While VPNs can be used for network connectivity, it's not how Kubernetes typically handles cross-node pod communication. Maintaining a full mesh of VPN tunnels would be extremely complex and resource-intensive, especially in large clusters. Overlay networks provide a much more scalable and manageable solution.