I am unable to connect the ESXi server to vcenter. How does vcenter communicate with each ESXi  server? Please provide a solution.
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To Understand How Does Vcenter Communicate With Each Esxi Server Using VSphere
VMware vSphere is used to communicate between a vcenter and ESXi server.
A vSphere sends a host agent to vcenter server. In return to this, the vcenter sends a vcenter server agent to the host to run on it. When the host agent gets permission, it communicates to ESXi server.
Another method is that vcenter server sends its agent named vpxa to esxi host. This happens when ESXi host sends a request to the server. The vcenter server adds the host to its inventory. ESXi host agent, hostd communicates to vpxa and starts the hostd process, and hence further work can be done.