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Author Topic: Need help to configure DHCP relay  (Read 5959 times)

tolli

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Need help to configure DHCP relay
« on: December 06, 2014, 04:23:55 AM »

Hi

I am trying to configure DHCP relay on my DSR-250N. I want to relay clients that are connected wirelessly to DHCP servers on the wired network.

Let me explain my setup:

- I have configured VLANS, the wireless interface is on VLAN 5 and the DHCP servers are on the default VLAN
- VLAN 5 is 172.16.3.0/24 and VLAN 1 is 172.16.0.0/24
- The DHCP servers are running Windows Server 2012 R2 and are configured in DHCP failover mode (load balanced)
- I have configured scopes for the appropriate networks on the DHCP servers

My key questions are:

What is the "Relay gateway" field supposed to be? The documentation and help file does not contain very much information on this. Is this supposed to be the gateway for VLAN 5 or VLAN 1? Or is it supposed to be the IP address for the DHCP server(s)?

What mode should the ports be set to, trunk, general or access? And what VLAN memberships should be set on the ports involved?

I have tried a lot of different combinations, but the clients that are connected wirelessly are unable to obtain IP addresses.

Any help/replies is greatly appreciated.

Thank you.
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PacketTracer

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Re: Need help to configure DHCP relay
« Reply #1 on: December 06, 2014, 07:22:00 AM »

Hi,

I'm not a user of a DSR-250N, but from my understanding when looking at the relevant parts of the manual (pages 25-29) I would configure the following:

Add a VLAN 5 for your wireless subnet 172.16.3.0/24 - according to the top image on page 26:
  • VLAN ID: 5
  • Name: whatever you like
  • Captive Portal: irrelevant, because not supported by DSR-250N
  • IP Address: Inspired by what you see in the last picture at page 25, I would guess that you don't have to put just the network address 172.16.3.0 into this field, but the address of the routing interface of your DSR-250N on this VLAN instead! Hence lets assume it is 172.16.3.1 and this address is not part of the DHCP pool you configured at your DHCP servers for use on this new VLAN. This address then should also be the default gateway for DHCP clients on this VLAN and hence be configured via DHCP option 3.
  • Subnet Mask: 255.255.255.0
  • DHCP Mode: DHCP Relay, and if asked for the "Relay Gateway" it is the address of your DHCP server of course
  • Enable DNS Proxy: You probably will leave this off because you will deploy DNS server addresses via DHCP option 6

Inside "Network > VLAN Settings > Port VLAN" I would assign the corresponding Wireless WLAN the mode "Access" with PVID 5.

PT
« Last Edit: December 06, 2014, 07:58:05 AM by PacketTracer »
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tolli

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Re: Need help to configure DHCP relay
« Reply #2 on: December 06, 2014, 08:53:43 AM »

Hi and thanks for the reply.

I have configured VLAN 5 exactly like you describe, except for the default gateway, which is on 172.16.3.254. Options in the DHCP scope are also configured accordingly.

The relay gateway can only hold one IP address, which makes it hard to configure multiple DHCP servers. Currently I have disabled failover for this scope.

After I set the wireless interface to "Access" and the corresponding VLAN id, like you described, I actually see DHCP leases being assigned in the DHCP server console, but the devices are still not getting IP addresses.

I will try to install Wireshark and sniff the network traffic later, to see if I can understand whats going on. Are there any log files on the DSR that I can examine?

Btw, here is a link to the latest version of the manual: http://www.dlink.com/-/media/Business_Products/DSR/DSR%20250N/Manual/DSR-250N_Manual_EN_UK.pdf

Edit: Fixed a typo.
« Last Edit: December 07, 2014, 03:06:00 AM by tolli »
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PacketTracer

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  • Posts: 439
Re: Need help to configure DHCP relay
« Reply #3 on: December 06, 2014, 09:13:07 AM »

Hi,

so, to summarize for a common understanding:

172.16.3.254 is ...
  • ... the DSR-250N interface address on VLAN 5
  • ... the IP address of the DHCP relay acting on VLAN 5, which has to talk to your DHCP server via IP unicast
  • ... the default gateway to be used for WLAN clients and configured via DHCP option 3

Can your DHCP server reach this address (ping test)? The DHCP server either needs a default gateway = DSR-250N interface address on VLAN 1 (172.16.0.?) or a static route for 172.16.3.0/24 with next hop being the DSR-250N interface address on VLAN 1 (172.16.0.?).

My manual link refers to the latest firmware version 2.x, available in the US but not in the UK. The  main difference concerning the relevant configuration according to pages 23-28 of your UK version seems to be the need to explicitely enable "Inter VLAN Routing", leaving unanswered the question if this feature is implicitely enabled for default VLAN 1 (it should, to make your scenario work).

PT
« Last Edit: December 06, 2014, 10:57:05 AM by PacketTracer »
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