Aironet 1142N not passing DHCP server info

[Jorge Hernandez]

Hey guys,

I'm trying to get this AP to work but for some reason even though computers can connect to it, they don't get an IP from our DHCP server, what can I do to get it to work?

This AP is an autonomous AP and there are no controllers on the network.

If you need more info, let me know.

[weterry]

Please provide relevant parts of the show run-config (all of it if you can)....

As well, what vlans have the problem, and whether or not you can get a wired client in that vlan working.

Generally speaking, an autonomous AP just bridges traffic out the vlan....   so you have one of two likely problems:

1) the AP can't decrypt the data.  if WPA/WPA2 you wouldnt even associate though,  if WEP you'd associate and would appear to not have an IP address

2) the client vlan is not properly bridging from the radio to the ethernet interface (or there is no DHCP server or IP-Helper in that vlan to get the DHCP request to a DHCP server)

[Jorge Hernandez]

Well, I do have it using WPA2 and I don't like WEP, my AP has ip dhcp-server on it, but I guess because of WPA it won't even try to contact it.

Any fix/workaround for this problem? that doesn't include old hacked/cracked encryption method?

Thanks for the info though.

[Nicolas Darchis]

If your dhcp server is on the wireless client subnet, all should be fine. But if it's on another subnet, you need the clients gateway to be configured with ip helper address to forward dhcp broadcasts to the DHCP server.

[Jorge Hernandez]

OK we are getting there, they are in two different subnets, so I guess it would be the ip helper setup, where do I have to setup the ip helper address? the wireless device or the layer 3 switch where it is, if it is the 1142N device to what interface?

Thanks for all your help.

[weterry]

You'll want your IP Helper on the Layer 3 interface, just like you would for any other wired vlan without a DHCP server.    The AP is just going to bridge traffic into the vlan, and generally I wouldn't expect it to get involved with DHCP.

If this was a lightweight controller, then by default the WLC is doing DHCP-Proxy, so on the WLC you define the DHCP servers per vlan,  but that is not the case for what you've described (since I believe you are autonomous)

[Jorge Hernandez]

That's what I thought, all our Layer 3 switches have ip helper setup on them, but for some reason it's not working with the AP.

Any ideas?

[weterry]

Then I would say you most likely have a situation where the trunk config to the AP doesn't allow the vlan in question, or that vlan isn't getting through the DS to the L3 network.

If you absolutely believe everything is configured correctly, then I'd  Span the Switchport the AP is connected to to prove once and foreall if a client DHCP request is  or is not being bridges to the DS as expected.

[ajmadriaga · Accepted solution]

Trunk the port that the AP is connected to and add "switchport trunk native vlan XXX".

[Jorge Hernandez]

Thanks for the advise, even though I did not do this, it let me check the configuration of the interface where the AP was connected and noticed that it had switchport security allowing just 1 IP for it, once I took it off it started to give IPs to the clients.

Thanks again.