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DHCP issues on Cisco WLC

Cisco Router, Firewall, VPN, SDM, ASA and Switch

DHCP issues on Cisco WLC

Postby guest » Mon Jun 11, 2012 8:28 am

quoted form Cisco

With the DHCP server address being (typical virtual IP address on a controller), the controller can intercept that packet and quickly respond to Windows.

This is also why the virtual IP address is the same on all controllers. If a Windows laptop roams to an AP on another controller, it will try to contact the virtual interface on the controller. Due to the mobility event and context transfer, the new controller that the Windows client roamed to already has all the information to respond to Windows again.

If you want to use the internal DHCP server on the controller, all you have to do is put the management IP address as the DHCP server on the dynamic interface you create for the subnet. Then assign that interface to the WLAN.

The reason the controller needs an IP address on each subnet is so it can fill in the DHCP gateway address in the DHCP request.

These are some of the points to remember when you configure DHCP servers for the WLAN:

1.The DHCP server IP address should not fall within any dynamic subnet that is on the controller. It will be blocked but can be overridden with this command:

config network mgmt-via-dynamic-interface on version 4.0 only
(command not available in version 3.2)2.The controller will forward the DHCP via unicast from its dynamic interface (in later code) using its IP address on that interface. Make sure that any firewall allows this address to reach the DHCP server.

3.Make sure that the response from the DHCP server can reach the controller's dynamic address on that VLAN through any firewalls. Ping the dynamic interface address from the DHCP server. Ping the DHCP server with a source IP address of the dynamic interface's gateway address.

4.Make sure the AP's VLAN is allowed on the switches and routers, and that their ports are configured as trunks so the packets (includes DHCP) tagged with the VLAN are allowed through the wired network.

5.Ensure that the DHCP server is configured to assign an IP address on the VLAN of the AP. You can also configure the WLC as a DHCP server. For more information on how to configure the DHCP server on the WLC, refer to the Using the GUI to Configure DHCP section of Cisco Wireless LAN Controller Configuration Guide, Release 5.0.

6.Verify that the controller's IP address on its dynamic interface will fall within one of the DHCP scopes on the DHCP server.

7.Finally, verify that you are not using a DHCP server that does not respond to unicast DHCP requests such as PIX.

If you cannot resolve your DHCP issue, there are 2 solutions:

•Try an internal DHCP server. Configure the DHCP server address on the dynamic interface to be the management IP address and then the DHCP internal pool. If the DHCP scope is enabled, it should work.

•Verify that there is no response to the DHCP request by sending in the output on the CLI (console or SSH) from these debugs:

0. debug mac addr <mac address>
1. debug dhcp message enable
2. debug dhcp packet enableThis should indicate that the DHCP packet was forwarded but the controller did not receive a response.

Finally, because of security on the controller, it is not recommend putting a VLAN or subnet on the controller that also contains the LAPs, unless it is on the management interface subnet.

Note: The RADIUS server or DHCP server must not be on any of the controller's dynamic interface subnets. Security will block the return packets that try to communicate with the controller.
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