WLC Dropping DHCP Discover Packets

[chris-kaiser]

Hello

I have following setup:

[Notebook]----wired connection----[Cisco Lynksys wlan ethernet bridge] )))))) wirelessConnection (((((  [AP]-----[switch]-----[Router]-----[WLC]

I use a seperat DHCP Server. so on the WLC the DHCP Proxy is disabled.

what i know is - the WLC should receive and forrward he DHCP Discover to the layer3 interface on the router, where the ip-Helper is configured. From there it should go to the DHCP Server.

BUT - here comes the problem -

The WLC Dropps the DHCP Discover.

*DHCP Socket Task: May 05 16:06:26.153: 88:ae:1d:b7:b7:c2 DHCP received op BOOTREQUEST (1) (len 308,vlan 253, port 13, encap 0xec03)
*DHCP Socket Task: May 05 16:06:26.153: 88:ae:1d:b7:b7:c2 DHCP option len (including the magic cookie) 72
*DHCP Socket Task: May 05 16:06:26.153: 88:ae:1d:b7:b7:c2 DHCP option: message type = DHCP DISCOVER
*DHCP Socket Task: May 05 16:06:26.153: 88:ae:1d:b7:b7:c2 DHCP option: 61 (len 7) - skipping
*DHCP Socket Task: May 05 16:06:26.153: 88:ae:1d:b7:b7:c2 DHCP option: hostname = Test (len 4)
*DHCP Socket Task: May 05 16:06:26.153: 88:ae:1d:b7:b7:c2 DHCP option: vendor class id = MSFT 5.0 (len
*DHCP Socket Task: May 05 16:06:26.153: 88:ae:1d:b7:b7:c2 DHCP option: 55 (len 13) - skipping
*DHCP Socket Task: May 05 16:06:26.153: 88:ae:1d:b7:b7:c2 DHCP options end, len 72, actual 64
*DHCP Socket Task: May 05 16:06:26.153: 88:ae:1d:b7:b7:c2 DHCP dropping packet (no mscb) found - (giaddr 0.0.0.0, pktInfo->srcPort 68, op: 'BOOTREQUEST')

Can anyone tell me why this happens?

[Sandeep Choudhary]

HI Chris,

First enable DHCP Proxy on WLC.

(Cisco Controller) >config dhcp proxy enable

and make sure that For DHCP proxy to work, a primary DHCP server must be configured on each controller interface that requires DHCP services.

To kow more about DHCO on WLC:

http://www.cisco.com/c/en/us/support/docs/wireless/4400-series-wireless-lan-controllers/110865-dhcp-wlc.html#Internal-DHCP

Regards

Dont forget to rate helpful posts

[chris-kaiser]

Hello Sandeep

thank you for your answer.

The DHCP Server is not on the Controller. it's on a other subnet. so DHCP proxy should not be used, i think?

the WLC is in DHCP Bridge mode.

[Sandeep Choudhary]

DHCP proxy can be used even if you have external DHCP server(I am using it.)

Just enable dhcp proxy and mention the dhcp server address on dynamic interface.

Check this link for more clarification:

http://www.cisco.com/c/en/us/support/docs/wireless/4400-series-wireless-lan-controllers/110865-dhcp-wlc.html#Internal-DHCP

Regards

Dont forget to rate helpful posts

[chris-kaiser]

Hello Sandeep

Tahnk you for your answer. i have read your link.

but i still dont get, why i need to enable dhcp proxy - bridging mode just works fine for me, only when the notebook is connected through a wlan bridge the dhcp discover is dropped.

[chris-kaiser]

enabled dhcp proxy mode globaly and on the interface. configured the primary and secondary dhcp server on the interface.

still getting the same dhcp discover drop.

[Justin Hallett]

I am having this exact issue, I'm upgraded to 7.0.220, I've tried proxy on and off, I've tried dhcp required on and off, and I've even tried dhcp server override set and off.  And every combo of them and I can not get it to pass dhcp at all, exact same messages as the OP.

Except mine start with 

*DHCP Proxy DTL Recv Task:

even when proxy is off.

My DHCP server is a cisco 3845, and I do not see the request getting to it to the WLC is dropping it and I can not figure out what "DHCP dropping packet (no mscb) found" means.  Specifically mscb so I'm out of options, but I really need to get this media bridge working.

Did the OP ever get this working? does anyone else have any ideas I can try?

[michaelblum]

older thread. but still.

The WLC uses the first DHCP message from a client to build a client list for mobility. Then drops the first packet. When the client sends the second DHCP discover it gets forwarded.

But this causes a delay in DHCP assignment.

Doesn't make sense, but I found this information a while back. (no links, sorry)

[DrKarol]

Hello everyone,

it is an old thread without a proper solution, perhaps someone can help.

I am having the same problem as described originally.  Connectivity: Printer ==ETH==> Cisco/LinksysWlanBridge ==WiFi==> Cisco 3802i (MobilityExpress) ==ETH===> Gateway/DHCPServer.

I can see DHCP discover packets arriving at my DHCP server and the server properly replies.  However, the reply packets never reach the Printer.  I had this working with an older enterprise 3Com access point, but when I recently changed to Cisco 3802i it does not work any more.  DHCP proxy or bridge mode on the interface does not make any difference.  There is no global DHCP setting on the Mobility Express WLC.

Best regards,

Karol