Description

This article provides information on how to associate the Cisco AP with WLC via DHCP Option 43 .

Symptoms

How to associate the Cisco AP with WLC via DHCP Option 43 .

Solution


What is the function of DHCP Option 43?

This feature helps the AP to associate with the WLC (Wireless controller) in a L3 environment (AP in one network and WLC in another network). The AP boots up, performs POST, and then sends a DHCP request. The switch should send out a DHCP offer, an IP address to use, a default gateway to use, and also the option 43 TLV . The TLV should contain the IP address of the WLC, with which the AP needs to be associated.

Topology :

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Cofiguration :

To configure the EX-series switch as a DHCP server and with option 43 for the Cisco wireless AP, perform the following procedure:
  1. Create the DHCP services on the EX switch by creating a DHCP pool: 
    user@switch> set system services dhcp pool 10.254.237.0/24
  2. Add the lowest and highest IP addresses in the DHCP pool address-range (optional): 
    user@switch> set system services dhcp pool 10.254.237.0/24 address-range low 10.254.237.1 high 10.254.237.254
  3. Add the router IP address to be provided with the IP address (optional): 
    user@switch> set system services dhcp pool 10.254.237.0/24 router 10.254.237.100
  4. Add the Wireless controller's IP address within the Hex string: 
    user@switch>set system services dhcp option 43 byte-stream “0xf1 0x04 0x0a 0xfe 0xed 0x01”
  5. Create a virtual interface to be associated with the DHCP pool: 
     user@switch> set interface vlan unit 0 family inet address 10.254.237.10/24
  6. Associate the virtual interface to a VLAN, to which the DHCP client’s machines will be connected: 
    user@switch> set vlan default l3-interface vlan.0

 Details about Option 43 for Cisco LWAP :

Add the Option 43 line with the following syntax:
set system services dhcp option 43 byte-stream “hexadecimal string”
The hexadecimal string is assembled as a sequence of the TLV values for the Option 43 sub-option - Type + Length + Value . Type is always the code 0xf1 sub-option. Length is the number of controller management IP addresses multiplied by 4 in Hex. Value is the IP address of the controller that is sequentially listed in Hex.

For example:

For this network, a controller with the management interface IP address of 10.254.237.1 is present. The type is 0xf1 . The length is 1 * 4 = 4 = 0x04 . The IP addresses translate to 0x0a 0xfe 0xed 0x01 . When the string is assembled, it yields f1040afeed01 . The command that is added to the DHCP scope is:
set system services dhcp option 43 byte-stream “0xf1 0x04 0x0a 0xfe 0xed 0x01”