DHCP relay not working

seth_turner_04 · 02-20-2008, 09:19 AM

I'm trying to get DHCP relay working on my 3Com 5500 switch, but not having any luck. Maybe there are some guys on here who run 3Com equipment that can help me out.

I have two subnets, a 10.30.x.x and a 20.1.1.x. A test machine is hooked up on the 20.1.1.x subnet and the dafault gateway of the interface is 10.20.0.1. My DHCP server is on the 10.30.x.x subnet and has the ip 10.30.0.24.

I setup the DHCP server with a 20.1.1.2 - 20.1.1.254 address range.

On the 3com 5500 switch, I entered the following commands.

[5500G-EI] dhcp enable
[5500G-EI] dhcp server 1 ip 10.30.0.24
[5500G-EI] udp-helper enable
[5500G-EI] interface vlan-interface 20
[5500G-EI-Vlan-Interface20] udp-helper server 10.30.0.24
[5500G-EI-Vlan-Interface20] dhcp server 1
[5500G-EI-Vlan-Interface20] ip address 20.1.1.1. 255.255.255.0

Whenever I do an ipconfig /release and /renew, I still do not receive an IP address from the DHCP server.

**Cellus** · 02-20-2008, 03:26 PM

Where is the switch located in the topology? If there is something like a router which is between the subnets and switch, the router may need to be configured to relay (ie. be a DHCP agent).

seth_turner_04 · 02-22-2008, 06:38 AM

Oops I seen I made a typo. The 20.1.1.x subnet has a default gateway of 20.1.1.1. I don't know why I put 10.20.0.1.

Anyways, each subnet interface is on a 3com 5500 switch. Each is a separate VLAN. The commands that I typed were done on the 3com 5500 switch. It is a layer 3 capable switch also.

Here's a pic of the topology.

seth_turner_04 · 02-26-2008, 09:38 AM

I did a packet trace on the server and the workstation at the same time. This is what it shows.

The DHCP server just shows normal traffic from the same physical subnet.
Source Destination Info
10.30.0.24 10.30.2.168 DHCP_ACK
10.30.0.24 10.30.0.103 DHCP_ACK
10.30.0.24 10.30.2.25 DHCP_ACK

This is what the workstation shows.
Source Destination Info
0.0.0.0 255.255.255.255 DHCP_DISCOVER
0.0.0.0 255.255.255.255 DHCP_DISCOVER
169.254.213.231 255.255.255.255 DHCP_INFORM
169.254.213.231 255.255.255.255 DHCP_INFORM

Does this mean that the switch is the problem still? It looks like a very simple setup so I don't see how it could be wrong.

02-20-2008, 09:19 AM	#1 (permalink)
seth_turner_04 Registered User Join Date: Sep 2006 Location: Missouri Posts: 67 OS: XP Pro SP3, Vista	DHCP relay not working I'm trying to get DHCP relay working on my 3Com 5500 switch, but not having any luck. Maybe there are some guys on here who run 3Com equipment that can help me out. I have two subnets, a 10.30.x.x and a 20.1.1.x. A test machine is hooked up on the 20.1.1.x subnet and the dafault gateway of the interface is 10.20.0.1. My DHCP server is on the 10.30.x.x subnet and has the ip 10.30.0.24. I setup the DHCP server with a 20.1.1.2 - 20.1.1.254 address range. On the 3com 5500 switch, I entered the following commands. [5500G-EI] dhcp enable [5500G-EI] dhcp server 1 ip 10.30.0.24 [5500G-EI] udp-helper enable [5500G-EI] interface vlan-interface 20 [5500G-EI-Vlan-Interface20] udp-helper server 10.30.0.24 [5500G-EI-Vlan-Interface20] dhcp server 1 [5500G-EI-Vlan-Interface20] ip address 20.1.1.1. 255.255.255.0 Whenever I do an ipconfig /release and /renew, I still do not receive an IP address from the DHCP server.

02-20-2008, 03:26 PM	#2 (permalink)
Cellus Moderator Networking Team Join Date: Aug 2006 Location: Canada Posts: 2,633 OS: Windows Vista Business SP1, Windows XP Professional SP3 My System	Re: DHCP relay not working Where is the switch located in the topology? If there is something like a router which is between the subnets and switch, the router may need to be configured to relay (ie. be a DHCP agent). __________________ TSF Networking Team HijackThis 5 Step Process Donate!

02-22-2008, 06:38 AM	#3 (permalink)
seth_turner_04 Registered User Join Date: Sep 2006 Location: Missouri Posts: 67 OS: XP Pro SP3, Vista	Re: DHCP relay not working Oops I seen I made a typo. The 20.1.1.x subnet has a default gateway of 20.1.1.1. I don't know why I put 10.20.0.1. Anyways, each subnet interface is on a 3com 5500 switch. Each is a separate VLAN. The commands that I typed were done on the 3com 5500 switch. It is a layer 3 capable switch also. Here's a pic of the topology.

02-26-2008, 09:38 AM	#4 (permalink)
seth_turner_04 Registered User Join Date: Sep 2006 Location: Missouri Posts: 67 OS: XP Pro SP3, Vista	Re: DHCP relay not working I did a packet trace on the server and the workstation at the same time. This is what it shows. The DHCP server just shows normal traffic from the same physical subnet. Source Destination Info 10.30.0.24 10.30.2.168 DHCP_ACK 10.30.0.24 10.30.0.103 DHCP_ACK 10.30.0.24 10.30.2.25 DHCP_ACK This is what the workstation shows. Source Destination Info 0.0.0.0 255.255.255.255 DHCP_DISCOVER 0.0.0.0 255.255.255.255 DHCP_DISCOVER 169.254.213.231 255.255.255.255 DHCP_INFORM 169.254.213.231 255.255.255.255 DHCP_INFORM Does this mean that the switch is the problem still? It looks like a very simple setup so I don't see how it could be wrong.