Just something I don't understand about network booting - can anyone clarify for me?

What I did was go through the script to enable the Terminal Server in Knoppix, and it went through happily.

Now, this script includes starting a DHCP service, but I already have a DHCP server on my network, and two DHCP servers is generally bad news, so I unplugged the other DHCP server from the network. This was also the Default Gateway for the LAN, but I was only testing PXE booting and wasn't going to need access to the outside world, so that's fine - or so I thought!

The client machine booted and got assigned an IP address, default gaeway etc. and that all went well. Then it got to the TFTP stage... and timed out. Tried several times, and every time the TFTP just timed out, and the machine carried on booting from its local HDD.

In desperation, I plugged the other DHCP server back into the network and tried again - bear in mind I now have TWO active DHCP servers on the LAN!

Hey presto, it booted and TFTP'd perfectly!!!

Repeated the exercise several times - with both servers on the LAN, it boots, with only the Knoppix server, the TFTP times out.

My question is this - the PXE booting procedure should be direct between the client machine and the Knoppix server machine - why doesn't it work when only these two machines are connected? Why does it seemingly need access to the outside world via the default gateway in order to boot over the local LAN? It shouldn't be going anywhere off the local LAN.

Just can't get my brain round this one!

Many thanks in advance!