topic DHCP requests from APs dropped by WLC in Wireless

DHCP requests from APs dropped by WLC

mathias — Sun, 04 Jul 2021 03:59:07 GMT

Hi, I'm trying to get a new 2504 WLC working with a AIR-LAP1142N-E-K9. I'm a newbee to Cisco WLC's so right now I'm just labbing with a simple setup but am unable to get the AP to recieve an IP address from the WLC.

I've configured port 0 as the management interface (untagged) with IP 10.105.8.120/16 and connected it to our local network, so I can succesfully administrate the WLC from my desktop. DHCP server: 10.105.8.120.

Then I've created a Dynamic AP Manager interface on port 3 (untagged aswell) with IP 172.21.200.254/24, DHCP server: 10.105.8.120.

And I've enabled DHCP proxy and configured a scope for 172.21.200.0/24.

Now when I connect the AP to port 3 on the WLC I would have expected it to get an IP from the internal DHCP server on the WLC, but it fails. With debug dhcp packet enable I get the following messages:

(Cisco Controller) >*DHCP Socket Task: Oct 25 04:41:14.603: 44:d3:ca:6b:10:b1 DHCP dropping packet from AP 44:d3:ca:6b:10:b1 received on port 3, vlan 0

*DHCP Socket Task: Oct 25 05:10:30.462: 44:d3:ca:6b:10:b1 DHCP received op BOOTREQUEST (1) (len 310,vlan 0, port 3, encap 0xec00)

*DHCP Socket Task: Oct 25 05:10:30.462: 44:d3:ca:6b:10:b1 DHCP option len (including the magic cookie) 74

*DHCP Socket Task: Oct 25 05:10:30.462: 44:d3:ca:6b:10:b1 DHCP option: message type = DHCP DISCOVER

*DHCP Socket Task: Oct 25 05:10:30.462: 44:d3:ca:6b:10:b1 DHCP option: 57 (len 2) - skipping

*DHCP Socket Task: Oct 25 05:10:30.462: 44:d3:ca:6b:10:b1 DHCP option: 61 (len 7) - skipping

*DHCP Socket Task: Oct 25 05:10:30.462: 44:d3:ca:6b:10:b1 DHCP option: 12 (len 16) - skipping

*DHCP Socket Task: Oct 25 05:10:30.462: 44:d3:ca:6b:10:b1 DHCP option: 55 (len 9) - skipping

*DHCP Socket Task: Oct 25 05:10:30.462: 44:d3:ca:6b:10:b1 DHCP option: vendor class id = Cisco AP c1140 (len 14)

*DHCP Socket Task: Oct 25 05:10:30.462: 44:d3:ca:6b:10:b1 DHCP options end, len 74, actual 66

I've also tried pointing DHCP server to 10.105.0.3 which is my local DHCP server, with the same result. Also tried changing the Dynamic AP interface to IP 10.105.8.121/16 so it's in the same subnet as my local subnet. Same result.

Tried "config network mgmt-via-dynamic-interface enable", but still with the same result.

So right now I'm pretty stuck. Why is the WLC dropping all my DHCP requests comming in on port 3?

WLC is running v7.0.116.0

DHCP requests from APs dropped by WLC

tuhpatel — Tue, 25 Oct 2011 13:36:55 GMT

Hello Mathias

Can you try to remove the newly created dynamic interface and enable the checkbox for Dynamic AP Manager on the Management interface itself ?

Also if you untag the interface on WC try using the native vlan option on the switch port on which the WLC is connected.

Let me know if this works for you,

Tuhin

DHCP requests from APs dropped by WLC

mathias — Tue, 25 Oct 2011 13:45:28 GMT

Hi Tuhin,

The problem is that the AP is connected directly to port 3, and the management interface is connected to port 0, so if I remove the dynamic interface, it won't reach the management net on port 3, right?

Port 0 is connected to a untagged (native vlan) switch port.

I'll give it quick test anyway to remove the dynamic interface.

My next step is to go down to the datacenter and connect the WLC and AP to a PoE switch instead, so I can try having the management VLAN extended to the AP over the switch instead, which I assume is the typical "simple" setup.

DHCP requests from APs dropped by WLC

mathias — Tue, 25 Oct 2011 13:54:52 GMT

Interesting! When I removed the Dynamic AP manager interface DHCP actually did start working. With the DHCP server still pointing to the WLC itself I got an error message that it couldn't find a sutable scope for 10.105.8.120, which is correct as I only had a scope for 172.21.200.0 defined in the WLC and now tried to relay from the management interface itself.

Changing to my local DHCP server 10.105.0.3, made the AP recieve an IP and start downloading the software correctly.

So, the question that remains is, why? Should I be able to put the APs on another VLAN / dynamic interface than the management interface?

DHCP requests from APs dropped by WLC

tuhpatel — Tue, 25 Oct 2011 14:03:08 GMT

Hey Mathias

Glad to knwo this worked out

It is not supported to use the AP on the backplane of the WLC

Please use a L2/ L3 switch

For a L2 swith the AP and WLC must be on the same VLAN iof the L2 switch

For AP:

config t

int gig 0/1

swithport access native vlan 1( for ex)

switchport mode access

no shut

For WLC :

config t

int gig 0/2

switchport mode trunk

switchport trunk encapsulation dot1q

switchport trunk native vlan 1

no shut

For L3 switch you can assign vlan interfaces :

http://www.cisco.com/en/US/products/ps6366/products_configuration_example09186a0080665cdf.shtml#wlc

Also here is the link to the discovery process:

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a00806c9e51.shtml

Thanks,

Tuhin

DHCP requests from APs dropped by WLC

mathias — Tue, 25 Oct 2011 14:07:47 GMT

Thanks a lot.

Just have to ask. What's the purpose of having PoE ports on the WLC, if it's not supported to connect APs directly to the WLC?

DHCP requests from APs dropped by WLC

tuhpatel — Tue, 25 Oct 2011 14:14:02 GMT

Anytime Mathias!

There are known bugs associated with directly connecting the AP off the backplane

The ports are used in case you were connecting to multiple switches or using Link Aggregation- LAG

This link shold serve as great reference:

http://www.cisco.com/en/US/docs/wireless/controller/7.0/configuration/guide/c70mint.html

Regards,

Tuhin

DHCP requests from APs dropped by WLC

Stephen Rodriguez — Tue, 25 Oct 2011 14:37:47 GMT

To clarify, the 2504 does not support LAG.

The ports are there to be used by the AP, but there are significant issues with the ARP code in the 2100/2500 series WLC that make using the ports a bad idea.

WNBU is still working on getting this code fixed.

HTH,

Steve

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DHCP requests from APs dropped by WLC

tuhpatel — Tue, 25 Oct 2011 15:01:18 GMT

That is correct

Thanks Stephen

DHCP requests from APs dropped by WLC

Vinay Sharma — Tue, 03 Jan 2012 09:36:36 GMT

Thanks Tuhin for sharing useful information on CSC. 5+

Regards,

Vinay Sharma

Community Manager