DHCP Relay Agent

Hi all,

 

Just thought I'd write a quick little ditty about the DHCP relay agent and some fun and games I've been having with it whilst studying.

My main network is 192.168.1.x/24, on it is my DC/DNS/DHCP server with a few other bits and bobs plus my ADSL modem/router. on that network is a server called "rras" who has 2 nics, one is 192.168.1.42 and the other is 172.16.10.2/16 which is connected to another virtual switch. the only thing connecting the 2 networks is the rras box. on the 172.16 side is one client PC. rras has been set up on the server and also the DHCP relay agent has been added to the 172.16 card and pointed to my DHCP server of 192.168.1.8 .
The DHCP server has 2 scopes for both subnet's all working fine. my ADSL modem/router also has a static route added to direct traffic bound for 172.16 to the 192.168.1.42 card. The "RRAS" server has also been added to the "rras and ias" group in Active directory.

Now the thing that has been bugging me is that the client PC has not been picking up any IP addresses. When I ran network monitor on both network cards of the rras server I saw discover packets being sent by the client, and I saw them popping out the other side of my network and I also sensibly enough saw offer packets being sent back to the 192.168 card on rras. What I wasn't seeing is the offer packets  popping out on the 172.16 subnet so that the DHCP process could continue.

 

This was really weird and I couldn't figure out why it wasn't working then all of a sudden it seemed to kick in and started working. I couldn't tell you why it suddenly started working other than I added the 172.16 scope to the DHCP server about an hour previous so I don't know whether there is naturally a delay in the relay agent working with the DHCP server.

 

The other thing that I found abit annoying in all my study material (3 books, mspress,exam cram and sybex) is that they don't say specifically which network card you should add the DHCP relay agent too. I assumed that it would need to go on the card attached to the subnet without a DHCP server but it was not specified. So whilst I was having this trouble I was wondering if I needed to add the relay agent to both cards!? Luckily it started working before I had tried that out.