Issue with DHCP Service using Windows 2012 R2 Data Center wtih Cisco 1841 ISR as a Relay Agent

[gregory.brugger]

Hello,

I am having troubles getting my DHCP Server to respond to DHCP Requests on another Network. The server has 3 NIC cards (2 Wired and One Wireless) but I only use 2 of the 3 connect (1 Wired and 1 Wireless). The Server currently has DNS and DHCP roles installed on it and DNS requests on both NIC Cards work Flawlessly, but I am only able to get the sever to respond to DHCP Requests on one NIC Card (Wireless).

The DHCP Scope is configured as follows: 192.168.1.0 /24.

The Wireless NIC Card is configured with 192.168.1.1 /24 IP Address and the Wired NIC is configured with 192.168.2.1 /24.

When I disable the Wireless NIC Card the other NIC Card with IP address of 192.168.2.1 /24 will not respond to DHCP Requests. I also run a Cisco 1841 ISR router on my network and also configured the Router to act as a rely agent with the IP Helper-Address command. I installed Wireshark on the Server and the Client and after installing I verified that the router is forwarding the messages to the unicast address of the server (192.168.2.1 /24) and the server is indeed receiving the DCHP Discover Messages (per captures displayed from Wireshark), but the Client does not ever receive a DHCPOffer (per wireshark) back from the server.

I am baffled and completely lost as what to do to fix this issue. I'd like the server to be located in another network using the 192.168.2.1 /24 address and assign IP's to the 192.168.1.0 /24 network but the server will not respond to any client requests.

Has any one ever ran into this issue similar to this before? If so, how did you resolve it?

It was always to my knowledge that DHCP Servers will respond to DHCP unicast requests and assign an address from a scope where the delivery method was from the default gateway of that network, but this time this isn't the case apparently.

For example: DHCP Server at IP address 192.168.2.1 /24 receices DHCPDiscover Packet from router with IP of 192.168.1.254 /24 so the DHCP Server assigns an IP Address out of the 192.168.1.0 /24 Network since this scope is set with the matching default gateway of the IP address in the DHCP Discover message.

Any help is greatly appreciated!

[gregory.brugger]

Hello I managed to resolve this issue. I created a mirrored port on my Cisco switch to mirror all traffic between the VLANs. I noticed that the server was indeed receiving the DHCPDiscover messages but did not do an offer. The server has 3 NIC cards (2 wired and 1 wireless). I had the RJ45 Ethernet cable plugged into NIC 2 instead of 1 and for some reason the server would not bind with the static address that was configured to it. Finally, I unplugged the Ethernet cable from NIC and put it into NIC 1 and the server was able to bind with the Static IP address given to it (still not sure why it wouldn't do it with the other NIC but it wouldn't). I noticed this when I went into Server Manager >> Tools >> DHCP >> Right Clicked "DHCP Server" >> Selected Add/Remove Server Bindings. The second NIC card was not appearing only the Wireless NIC Card was appearing with the static IP of the second network. Now when I disable/enable both NICs, they both respond to DHCP requests as they should.

Hopefully this will help any one who has a similar issue and runs into this. This is what fixed it for me.

[Richard Burts]

I am glad that you were able to find the solution to your own problem. Thank you for posting back to the forum to tell us that it was solved, and what the problem turned out to be. This may very well help some other reader of the forum who may run into a similar issue.

HTH

Rick