We have learned about ESXi, vCenter and different vSphere components.
Now let's understand how exactly vCenter and ESXi communicate with each other.
Above figure shows the high level overview of how this communication happens.
When you first logged in to the vCenter Server system through the vSphere Web Client, vCenter Server passes commands to the ESXi host through the vpxa process. Vpxa communicates with the ESXi host agent named hostd. If you are using the vSphere Client to communicate directly with an ESXi host, communications go directly to the hostd process and the vCenter Server database is not updated.
By default, the vSphere Client uses ports 80 and 443 to communicate with vCenter Server and ESXi hosts.
What is hostd, vpxa and vpxd?
1. Hostd
hostd is a daemon running on ESXi host.It is a management service which is the main communication channel between ESXi hosts and VMkernel. If the hostd service is not running on the ESXi host, you won't be able to communicate with the ESXi host, directly or from the vCenter.
It keeps the track of all the VMs that are registered on that host, the storage volumes visible by the host, status of all virtual machines etc. Most all commands or operations come down from VC through it. i.e, powering on a VM, VM vMotion, VM creation, etc.
2. vpxd
vpxd is a daemon running on vCenter server. If this service is not running you won't be able to access the vCenter server. 3. vpxa vpxa is an agent running on the ESXi host. When we add a ESXi host to vCenter Server, it gets install on the ESXi host. This vpxa service acts an an intermediary service between vpxd(service running on vCenter) and hostd(service running on ESXi) to relay the tasks to perform on the host.
For more information on ports required to access the vCenter,ESXi and other network components, please refer this VMware KB. I hope people who are new to VMware would find this blog post useful.
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