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This section covers the following topics:
A PXE workstation will broadcast for DHCP information and receive a response from both the DHCP and Proxy DHCP services. The workstation will then download the preboot client and check for work with the Transaction server.
No special configuration is required in a LAN environment. Make sure that the DHCP server is allocating IP addresses and that the Proxy DHCP and TFTP services are started.
In a wide area network environment, the PXE workstation is usually separated from the Proxy DHCP and DHCP servers by one or more routers. The PXE workstation will broadcast for DHCP information, but by default the router will not forward the broadcast to the servers, causing the PXE session to fail.
There are two ways to make PXE work correctly in a WAN:
A DHCP broadcast is a UDP broadcast frame with the destination port set to 67.
With this solution, the PXE workstation will receive a response from a Proxy DHCP server located on its subnet, but it is redirected to a single central Transaction server to check for work to do.
In a VLAN environment, the PXE workstation is logically separated from the Proxy DHCP server and the DHCP server by a switch. At the IP level, this configuration looks very similar to a traditional WAN or routed environment.
Configuration of a VLAN is basically the same as a WAN. Configure the VLAN that the PXE workstation belongs to with a DHCP relay agent or IP helper to forward DHCP broadcasts to the DHCP server and the Proxy DHCP server, or place a Proxy DHCP server and a TFTP server in each VLAN to serve the PXE workstations directly.
Some network devices filter network traffic that passes through them. PXE makes use of several different types of traffic, and all of these must be able to pass through the router or switch successfully for the PXE session to be successful. The PXE session will use the following destination ports:
| Component | Port |
|---|---|
DHCP and Proxy DHCP |
UDP Port 67 |
TFTP |
UDP Port 69 |
RPC Port Map Service |
UDP Port 111 |
Transaction Service |
UDP Port 18753 |
Spanning tree protocol (STP) is available on certain switches and is designed to detect loops in the network. When a device (typically a network hub or a workstation) is patched into a port on the switch, the switch indicates to the device that the link is active, but instead of forwarding frames from the port to the rest of the network, the switch checks each frame for loops and then drops it. The switch can remain in this listening state from anywhere between 15 to 45 seconds.
The affect of this is to cause the DHCP requests issued by PXE to be dropped by the switch, causing the PXE session to fail.
It is normally possible to see that the STP is in progress by looking at the link light on the switch. When the workstation is off, the link light on the switch is obviously off. When the workstation is turned on, the link light changes to amber, and after a period changes to a normal green indicator. As long as the link light is amber, STP is in progress.
This problem will only affect PXE or preboot clients that are patched directly into an Ethernet switch. To correct this problem, perform one of the following:
Once the problem is resolved, the link light on the port should change to green almost immediately after a workstation connected to that port is turned on.
Information about STP and its influence on DHCP can be found at Using PortFast and Other Commands to Fix End-Station Startup Connectivity Problems.
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