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  Asked: Mar 20 2007   3:56 PM GMT
  Asked by: dimchik

DHCP relay Agent in DELL switches


Routers, Microsoft Windows, OS, Servers, SQL Server, Security, Desktops, Management, Networking, Dell, Networking Services, DNS, DHCP, Switches

I have windows 2003 servers and Dell 62XX series switches (level 3 switches) and Vlans. As I understand in order for a computer to be able to receieve an IP Address via dhcp I need relay agent. This series of switches supports DHCP Relay my question is how would i assign computer to a specific subnet. EX. I have computer_5 on Vlan 5 which should be in subnet 172.16.5.X In windows 2003 server I defined scopes for all my VLANs
172.16.5.x Vlan 5
172.16.6.x Vlan 6
172.16.7.x Vlan 7

How would DHCP server know what computers in which VLAN or scope without using (MAC Address reservation) reservation for each computer.

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Last Answered: Mar 20 2007  4:56 PM GMT  by jmweber




Provided that you have your VLAN setup correctly, and furthermore that you have your DHCP forwarding correct, then the DHCP REQ comes from a LAN port on subnet 172.16.5.x, and the TCP packet has headers that identify the route taken to get to the dhcp server.
The dhcp server assigns addresses per that packet address.
You need do nothing more than make sure your infrastructure is correct.
-John
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astronomer  |   Mar 21 2007  12:12PM GMT

The relay agent needs to be on a device where it can receive the DHCP request, (broadcast), from the client. This is normally done on the router but doesn’t have to be. The relay agent uses the IP of the interface receiving the request to determine the subnet the request is on. The appropriate subnet IP is added to the request before it is sent to the DHCP server. This way, the DHCP server knows what subnet the request is from. The DHCP server then offers an IP from a scope appropriate for that subnet and sends it back to the DHCP relay device.

Here is how cisco describes the process:
1. The DHCP client generates a DHCP request and broadcasts it on the network.

2. The DHCP relay agent intercepts the broadcast DHCP request packet and inserts the relay agent information option (option 82) in the packet. The relay agent information option contains the related suboptions.

3. The DHCP relay agent unicasts the DHCP packet to the DHCP server.

4. The DHCP server receives the packet and uses the suboptions to assign IP addresses and other configuration parameters and forwards them back to the client.

Another option for a network as small as yours is to add interfaces to the DHCP server so it can sit on every subnet. This allows the DHCP server to directly respond to the request broadcasts without needing a DHCP relay.

Does this answer all of your concerns?
rt

 

Buddyfarr  |   Feb 21 2008  6:14PM GMT

jmweber is correct, if the DHCP server is connected to this switch and doesn’t have a router in between it and that switch the vlans are on then you do not have to go any further. the dell switch should forward the DHCP request onto the server showing that it came from vlan 172.x and the DHCP server should give out addresses accordingly.