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How Client VMs stream vDisk from Citrix PVS Server

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Citrix Provisioning Server (PVS) streams the contents of the vDisk to the Client VMs (target device) on demand, in real time. The Client VMs boot directly across the network and behaves as if it is running from its local drive. This article talks about various steps involved in network boot and how the vDisk is streamed.

Steps:

Let's say, a Client VM is powered ON and it is configured to boot from Network.

Since it is configured to boot from network, it gets IP address from DHCP Server.

DHCP IP Address detailsAfter getting IP Address from DHCP, Client sends broadcast request for PXE servers in the network. In general both PXE and TFTP services are provided by the PVS server itself.

Let’s say there are 2 PVS servers in the network having PXE service enabled. Both the PVS servers will respond to the client's broadcast request

Client contacts the PXE server from which it receives the response first. PXE session is established

PXE service on PVS server sends the information of TFTP server IP address and name of the Bootstrap Program file to download from TFTP. The only use of PXE is to provide TFTP server details to clients.

Bootstrap program is a small program which initializes the streaming session between Client and PVS server.

Client establishes the TFTP connection to PVS Server and downloads the Bootstrap Program. The only purpose of TFTP service is to provide Bootstrap program to clients.

Bootstrap Program and PVS Server IP detailsBootstrap program file contains the information of which PVS servers to contact for vDisk download. In Citrix terminology, these servers are called Login servers (shown in above screenshot).

Bootstrap program contacts the PVS servers for vDisk. PVS checks its database for the target device mapped to client's MAC address and the vDisk assigned to it.

Client establishes the streaming session to PVS Server. Client boots up with the OS image in the vDisk file

Connecting to PVS Server for vDiskJust to summarize, PXE is used for getting the TFTP Server IP and bootstrap file name details by the clients and TFTP is used for downloading bootstrap program file.

Is there a way to eliminate PXE from the communication process?

Yes, by specifying TFTP Server IP Address and bootstrap file name in DHCP Scope options on DHCP Server. DHCP Scope options 66 and 67 can be used. DHCP server provides TFTP details to the clients along with dynamic IP address. Client VMs will directly contact TFTP server and download the bootstrap program.

Is there a way to eliminate both PXE and TFTP from communication process?

Yes, by using Boot Device Manager (BDM) both PXE and TFTP can be eliminated from communication process. Using Boot Device Manager utility on PVS Server, bootstrap file can be written into a ISO image, USB flash drive, or local hard drive. Client VMs need to be configured to boot from ISO image file. As ISO image file itself has bootstrap program, clients doesn't use PXE or TFTP.
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