Preboot Execution Environment (PXE) is a protocol that enables a client machine to boot from its network interface and connect to a server resource on the network to retrieve a boot-file program and load an Operating System. When a workstation start with PXE boot, the PXE client sends a DISCOVER packet (broadcast) to the entire domain to search for a DHCP server to get an IP address. If it doesn’t find one, it tries a few times and then it times out. In Citrix PVS technology the target device is just a PXE-enabled machine. In most of the subnetted network the target device fails to obtain an IP while DISCOVER and cannot reach a DHCP server where PVS targets are on one physical segment and DHCP resources are on a another one. Because of a limitation in PXE, the broadcast packet never reaches the other segments. So to overcome this issue there is a feature called DHCP Relay (also known as IP Helper for specific vendors) that can be enabled with a simple command on the router in order to make it “listen” to PXE packets and “relay” them to the next-next subnets until they reach a DHCP destination.
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