WDS and multiple subnets subnets

Question

Goal: To allow my one WDS server on Subnet A to accept client PXE boots from subnets B, C, D, and E.

Setup: Currently, WDS is installed and configured with default settings on my Deployment (Server 2008R2) server on subnet A. I have computers on Subnet B-E that need access to the WDS server for deployment purposes (Using MDT2012) The Deployment server does not have any other roles other than File Services and WDS.

Problem: We are in a locked down environment. We have our own locked away subnets, but we do not have access to create any VLANs at all. We also do not have the ability to make changes to the switches at all. I can make changes to our DHCP (I think) and request DNS modifications.

Hope/What I would love from an answer: A how to of configuring WDS to work across multiple subnets, while touching as few servers as possible...We work in a small environement, and I am rather new to server work, so I would perfer to not mess with DHCP if possible. Anyway this can all be done in WDS settings?

Solution

The short answer is no, you can't do what you're asking without access to the network configuration. The PXE bootstrap broadcasts a DHCPDISCOVER packet with PXE-specific options on UDP port 67, which is not normally routed. Because of this, the PXE server must be located on the same subnet unless you configure your network for forward UDP traffic.

There are only two ways to serve PXE requests on a multiple subnets using one server:

1. Multihome your WDS server via additional NICs (or VLAN tagging via a single NIC if your NIC and switch support it) or,
2. Configure your network to forward UDP broadcasts (in Cisco IOS this is done using the ip helper-address command)

Unfortunately, either method will require access to network equipment, although the first may only require access to a port on the appropriate VLAN if you use multiple NICs.

Comments

[Ashmeet Singh]

To be clear, it is not sufficient to have a separate DHCP server with a scope on the subnet in question. Per Microsoft (technet.microsoft.com/en-us/library/cc766320(WS.10).aspx) WDS must also receive the UDP traffic from the client in order for WDS to function. This only possible if WDS is on the same subnet or IP helper tables are configured.