vCenter Server communicates with the ESX/ESXi host agent through the VMware vSphere API.

When you first add a host to vCenter Server, vCenter Server sends a vCenter Server agent to run on the host. As Host Agent shows, that agent communicates with the host agent.

Host Agent
This image shows how the vCenter agent communicates with the host agent.

The vCenter Server agent acts as a small vCenter Server to perform the following functions:

Relays and enforces resource allocation decisions made in vCenter Server, including those that the DRS engine sends.

Passes virtual machine provisioning and configuration change commands to the host agent.

Passes host configuration change commands to the host agent.

Collects performance statistics, alarms, and error conditions from the host agent and sends them to the vCenter Server.

Allows management of ESX/ESXi hosts at different release versions.