incorrect DHCP server address to wireless client

[amar_5664]

Guys,

I have deployed a centralized wireless architecture with 2 x wisms in each of our DCs. I just noticed the clients are receiving incorrect DHCP server address, please note there is no issue with connectivity, i just want to know why incorrect DHCP server address is handed out to the clients.

The DHCP server address received is apparently virtual interface address of the WiSM. i am running WiSM firmware 7.0.116.0

Any information is appreciated.

Cheers

AP

[Stephen Rodriguez · Accepted solution]

This is the expected behavior of a wireless network when you have DHCP proxy enabled.

If you want the client to show the real DHCP server you could disable proxy and add an ip helper to the L3 interface.

Note you can't disable proxy if the WLC is a DHCP server for any WLAN.

Steve

Sent from Cisco Technical Support iPhone App

[amar_5664]

Thanks Steve, i just came across that document. Appreciate your help.

[George Stefanick · Accepted solution]

Amar,

So you know, if DHCP proxy is enabled (which it is by default) there us no need for  dhcp ip helpers on the vlan. The WLC will unicast the dhcp request from the WLC dynamic interface. I participated in a white paper with Marcus Burton from CWNP.

You can read a little bit about it here. This may help as well...

http://www.cwnp.com/cwnp_wifi_blog/dhcp-for-wireless-lan-clients

[amar_5664]

This is great, thanks George.

[George Stefanick]

DHCP proxy is pretty usless in my mind. It was something that lingered from the old airespace days. In fact if you ask many cisco se's they will tell you they wished they didnt default to proxy. Also, if you ever use cisco ISE, you will need to disable proxy, becuase you will need to configure a helper pointing to the ISE box for DHCP finger printing.

Steve points out, if you use the WLC as a DHCP server it is a requirement to have it on. But we all agree here on CSC (Steve, Leo, Blake, myslef), that the DHCP server on a WLC is not robust and is problematic at times.

[weterry]

Keep in mind, there hasn't always been the ability to bridge dhcp to DS.   The old disable dhcp proxy, did just that, disabled "Proxy". It still relayed.    Now disable dhcp proxy disables the proxy and the relay and allows for dhcp bridging to DS.

FYI, since 7.0.98.0 the WLC Configuration Wizard specifically asks for DHCP Bridging Behavior (Configure DHCP Bridging Mode [yes][NO]:)

So at least now you are specifically asked what to do about DHCP bridging (bridging = proxy off).