Network foo: How can I get a DHCP client to reach the DHCP Server thru Access Point in another Network?

November 27, 2007 8:39 AM

I have a DHCP client wireless device that needs to connect to a DHCP server. This device only works with DHCP. The problem is the device will connect through an access point that is in a different network.

The access point connects to Cisco switch on vlan 2, the server is on vlan 3. The wireless device needs to be in vlan 3

DHCP Client (10.1.x.x)-> AP (10.2.x.x.) ->Cisco ->Server (10.1.x.x)

What can I do to the Cisco switch to get the device to reach vlan 3?

posted by nivekraz to Technology (4 answers total)

I'm making some assumptions here, but I think your issue here is with the AP.

Let's start with the Cisco. Off of it, you have two vlans:

VLAN3 - 10.1.0.0/16
VLAN2 - 10.2.0.0/16

You've plugged your AP into VLAN2, and configured it such that it's Internet/WAN port is on VLAN2, and it's wireless subnet is configured as 10.1.0.0/16, which is what it's DHCP server is handing out addresses for.

That's not going to work. I think what you _really_ want here, is to configure your AP this way:

Internet/WAN port: Disabled.
Wireless Subnet: 10.2.0.0/16

Instead of plugging the AP into the switch via it's WAN port, use one the AP's switch ports. This'll logically bridge your Wireless onto VLAN2.

..Or did you mean something else?
posted by Laen at 9:38 AM on November 27, 2007

I think you need to configure the switch as a DHCP relay. Look for help on "ip helper-address".
posted by dreadpiratesully at 9:42 AM on November 27, 2007

Although simply getting an address from the DHCP server on vlan3 might not do what you need it to do. The device would still be on vlan2.
posted by dreadpiratesully at 9:48 AM on November 27, 2007

dreadpiratesully has it. You want to tell the router that serves Vlan2 to forward DHCP requests to the DHCP server. ip helper-address is indeed the correct command. Be aware that this will forward ALL udp broadcasts, not just DHCP. You will also need to configure the DHCP server to now service both it's native Vlan (3) and the newly added Vlan2.

Alternatively, you could setup the router that serves Vlan2 to act as a temporary DHCP server to simply get the device online to configure it with a static IP. Routergod has an amusing tutorial for this that's actually far easier to read than the corresponding cisco documentation.

Do you not have access to the router that serves both of these Vlans? Then get shady, buy a used home router on craigslist, configure it's internal (LAN) interface to match the IP range used by Vlan 2 and tell it that it's a DHCP server. Make sure it's not using the *actual* router's IP as it's own - that would cause problems obviously. Then plug it LAN-side-in to a wired port on your network and set that port to Vlan2. Voila. instead guerilla DHCP server on the wireless LAN for <$50. Also barricade your office door if you're not the net engy because he or she will hunt you down.
posted by datacenter refugee at 5:12 PM on November 27, 2007