Configure DHCP OPTION 43 for Lightweight Access Points

Introduction

This document describes how to use DHCP Option 43 for lightweight access points.

Prerequisites

The document provides sample configurations for DHCP Option 43 for lightweight Cisco Aironet access points (LAPs) for these DHCP servers:

• Microsoft Windows 2008 Enterprise DHCP Server

• Cisco IOS^® DHCP Server

• Linux Internet Systems Consortium (ISC) DHCP Server

• Cisco Network Registrar DHCP Server

• Lucent QIP DHCP Server

When a Cisco Wireless Unified architecture is deployed, the LAPs can use a vendor-specific DHCP Option 43 to join specific Wireless LAN Controllers (WLCs) when the WLC is in a different subnet than the LAP. Refer to Wireless LAN Controller and Lightweight Access Point Basic Configuration Example and Lightweight AP (LAP) Registration to a Wireless LAN Controller (WLC) for information on how to configure an access point (AP) to join a WLC.

Requirements

Cisco recommends that you have knowledge of these topics:

• Basic knowledge on Cisco Unified Wireles Network (CUWN)

• Basic knowledge of DHCP

Components Used

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Background Information

Vendor Specific DHCP Options

RFC 2132 defines two DHCP Options that are relevant to vendor specific options. They are Option 60 and Option 43. DHCP Option 60 is the Vendor Class Identifier (VCI). The VCI is a text string that uniquely identifies a type of vendor device. This table lists the VCIs used by Cisco APs:

^¹ Early manufactured 9136 can ship with a software that send "Cisco AP" in the option 60. This was fixed in 17.8 and later software versions.

⁶Any 2700/700/1530 Series AP that runs 7.6 or later

⁷Any 3700 Series AP that runs 7.6 or later software

⁸1540s that run pre-FCS manufacturing code can use "Cisco AP c1560"

⁹1815s that run pre-FCS manufacturing code can use "Cisco AP c1810"

Also, see the the Cisco Wireless Solutions Software Compatibility Matrix.

Option 60 is included in the initial DHCP discover message that a DHCP client broadcasts in search of an IP address. Option 60 is used by DHCP clients (LAPs in this case) in order to identify itself to the DHCP server.

If the access point is ordered with the Service Provider option (AIR-OPT60-DHCP selected), the VCI string for that access point is different than those listed previously. The VCI string includes the ServiceProvider option. For example, a 1260 with this option returns this VCI string: Cisco AP c1260-ServiceProvider.

Leveraging option 60 is not a required and your DHCP pools can return option 43 for any type of clients. However, the DHCP server can be programmed in order to return one or more WLAN controller management interface IP addresses based on the VCI of the AP. In order to do this, program the DHCP server to recognize the VCI for each access point type, and then define the vendor specific information.

On the DHCP server, the vendor specific information is mapped to VCI text strings. When the DHCP server sees a recognizable VCI in a DHCP discover from a DHCP client, it returns the mapped vendor specific information in its DHCP offer to the client as DHCP Option 43. On the DHCP server , option 43 is defined in each DHCP pool (Scope) that offers IP address to the LAPs.

RFC 2132 defines that DHCP servers must return vendor specific information as DHCP Option 43. The RFC allows vendors to define encapsulated vendor-specific sub-option codes between 0 and 255. The sub-options are all included in the DHCP offer as type-length-value (TLV) blocks embedded within Option 43. The definition of the sub-option codes and their related message format is left to the vendors.

When DHCP servers are programmed to offer WLAN Controller IP addresses as Option 43 for Cisco Aironet LAPs, the sub-option TLV block is defined in this way:

• Type - 0xf1 (decimal 241).

• Length - Number of controller IP addresses * 4.

• Value - List of the WLC management interfaces, typically translated to hexadecimal values.

The semantics of DHCP server configuration vary based on the DHCP server vendor. This document contains specific instructions on the Microsoft DHCP server, Cisco IOS DHCP server, Linux ISC DHCP Server, Cisco Network Registrar DHCP server, and Lucent QIP DHCP Server. For other DHCP server products, consult the vendor documentation for instructions on vendor specific options.

Configure

Note: Use the Command Lookup Tool (registered customers only) in order to obtain more information on the commands used in this section.

Microsoft DHCP Server

This section describes the configurations necessary on the Microsoft DHCP server in order to use DHCP Option 43 for WLAN Controller discovery.

Cisco Lightweight Access Points

The method described in the previous section can be used if you have multiple device types on the same scope and you want them to receive different WLC IP addresses via Option 43. But, if all of the DHCP clients in the scope are Cisco IOS APs, you can use this procedure to define DHCP Option 43.

Before you begin, you must know this information:

• Option 43 sub-option code

• Management IP address(es) of WLAN controller(s)

Complete these steps in order to define DHCP Option 43 on the Windows DHCP server:

1. In the DHCP Server scope, right-click Server Options and choose Configure Options.

   

2. On the General tab, scroll to Option 43 and check the 043 Vendor Specific Info check bo

3. Enter the Option 43 sub-option in hex.

   Note: TLV values for the Option 43 suboption: Type + Length + Value. Type is always the suboption code 0xf1. Length is the number of controller management IP addresses times 4 in hex. Value is the IP address of the controller listed sequentially in hex. For example, suppose there are two controllers with management interface IP addresses, 192.168.10.5 and 192.168.10.20. The type is 0xf1. The length is 2 * 4 = 8 = 0x08. The IP addresses translates to c0a80a05 (192.168.10.5) and c0a80a14 (192.168.10.20). When the string is assembled, it yields f108c0a80a05c0a80a14. The Cisco IOS command that is added to the DHCP scope is option 43 hex f108c0a80a05c0a80a14.

4. Click Apply and then click OK.

   Once you complete this step, the DHCP Option 43 is configured and the DHCP server sends the option 43 to the LAPs.

   

Cisco IOS® DHCP Server

Cisco Aironet APs (Cisco IOS)

Complete these steps in order to configure DHCP Option 43, in the embedded Cisco IOS DHCP server, for all Cisco Aironet APs that run Cisco IOS. This includes all APs except for the VxWorks 1000 Series (see the next section) and the 600 Series OEAP which does not use Option 43.

1. Enter configuration mode at the Cisco IOS CLI.

2. Create the DHCP pool, which includes the necessary parameters such as the default router and server name. This is an example DHCP scope:

   ip dhcp pool <pool name>
   network <ip network> <netmask>
   default-router <default-router IP address>
   dns-server <dns server IP address>

3. Add the Option 43 line with this syntax:

   option 43 hex <hexadecimal string>

   The hexadecimal string in step 3 is assembled as a sequence of the TLV values for the Option 43 suboption: Type + Length + Value. Type is always the suboption code 0xf1. Length is the number of controller management IP addresses times 4 in hex. Value is the IP address of the controller listed sequentially in hex.

   For example, suppose there are two controllers with management interface IP addresses, 192.168.10.5 and 192.168.10.20. The type is 0xf1. The length is 2 * 4 = 8 = 0x08. The IP addresses translate to c0a80a05 (192.168.10.5) and c0a80a14 (192.168.10.20). When the string is assembled, it yields f108c0a80a05c0a80a14. The Cisco IOS command that is added to the DHCP scope is:

   option 43 hex f108c0a80a05c0a80a14

Linux ISC DHCP Server

The information in this section describes how the Linux ISC server is configured in order to return vendor specific information to lightweight Cisco Aironet Series APs. This example configures the Linux ISC server to return vendor specific information to the 1140, 1200, 1130 and 1240 Series Lightweight APs. This configuration can be modified and applied to other series of LAPs.

ddns-update-style interim;
     allow bootp;
     option space Cisco_LWAPP_AP;
     option Cisco_LWAPP_AP.server-address code 241 = array of ip-address;
     subnet 192.168.247.0 netmask 255.255.255.0 {
     authoritative;
     option routers 192.168.247.1;
     option subnet-mask 255.255.255.0;
     option domain-name "cisco.com";
     option domain-name-servers 192.168.247.2, 192.168.247.3;
     range dynamic-bootp 192.168.247.11 192.168.247.254;
     default-lease-time 300;

class "Cisco-AP-c1140" {

       match if option vendor-class-identifier = "Cisco AP c1140";
       option vendor-class-identifier "Cisco AP c1140";
       vendor-option-space Cisco_LWAPP_AP;
       option Cisco_LWAPP_AP.server-address 192.168.247.5; }

 class "Cisco AP c1200" {

   match if option vendor-class-identifier = "Cisco AP c1200";
   option vendor-class-identifier "Cisco AP c1200";
   vendor-option-space Cisco_LWAPP_AP;
   option Cisco_LWAPP_AP.server-address 192.168.247.55; }

  
 class "Cisco AP c1130" {

   match if option vendor-class-identifier = "Cisco AP c1130";
   option vendor-class-identifier "Cisco AP c1130";
   vendor-option-space Cisco_LWAPP_AP;
   option Cisco_LWAPP_AP.server-address 192.168.247.5; }

  
 class "Cisco AP c1240" {

   match if option vendor-class-identifier = "Cisco AP c1240";
   option vendor-class-identifier "Cisco AP c1240";
   vendor-option-space Cisco_LWAPP_AP;
   option Cisco_LWAPP_AP.server-address 192.168.247.5; }
   
 }

Cisco Network Registrar DHCP Server

 The Cisco Network Registrar DHCP server supports Vendor Specific attributes. Refer to the product documentation : https://www.cisco.com/c/en/us/td/docs/net_mgmt/prime/network_registrar/11-2/dhcp/guide/DHCP_Guide/DHCP_Guide_chapter_0110.html?bookSearch=true#task_jsq_sdr_1nb

Lucent QIP DHCP Server

This section provides a few tips for how to configure the Lucent QIP DHCP server in order to return vendor specific information to lightweight Cisco Aironet Series APs.

Note:For complete information and the steps involved, refer to the documentation provided by the vendor.

The DHCP Option 43 can contain any vendor specific information. The DHCP server passes this information in the form of a hex string to the clients that receive the DHCP offer.

On the Lucent QIP DHCP server, the vendor-specific information can be provided on the DHCP Option Template- Modify page. In the Active Options area, choose Vendor Specific Information, and enter the information in the Value field.

In order to include the controller IP addresses in the DHCP option 43 message, enter the information to the DHCP Option template in QIP as a single hex value: [ip hex].

In order to send more than one IP address with DHCP Option 43, enter the information to the DHCP Option template in QIP as a single hex value:[ip hex ip hex] and not [ip hex],[ip hex]. In this case, the comma in the middle causes problems for DHCP to parse the string passed from QIP.

For example, suppose there are two controllers with management interface IP addresses, 192.168.10.5 and 192.168.10.20. The type is 0xf1. The length is 2 * 4 = 8 = 0x08. The IP addresses translate to c0a80a05 (192.168.10.5) and c0a80a14 (192.168.10.20). When the string is assembled, it yields f108c0a80a05c0a80a14. On the Lucent QIP DHCP server, the hex string that needs to be added to the DHCP scope is:

[f108c0a80a05c0a80a14]

The hex string must be given within square brackets. The square brackets are mandatory. Once the DHCP option 43 is modified to reflect this value, the LAPs are able to find and register with the controller.

Verify

Use this section in order to verify your configuration.

The Output Interpreter Tool (registered customers only) supports certain show commands. Use the Output Interpreter Tool in order to view an analysis of show command output.

If you use 1130 /1200/1230/1240 Series LAPs, which have a console port, you can check that the WLC IP addresses are provided to the LAPs during DHCP IP address assignment. This is a sample output from a Cisco 1230 Series LAP:

*Mar  1 00:00:17.497: %LINEPROTO-5-UPDOWN: Line protocol on Interface 
      Dot11Radio1, changed state to down
*Mar  1 00:00:17.898: %LINEPROTO-5-UPDOWN: Line protocol on Interface 
      Dot11Radio0, changed state to down
*Mar  1 00:00:25.352: %DOT11-6-FREQ_USED: Interface Dot11Radio0, frequency 
      2447 selected
*Mar  1 00:00:25.353: %LINK-3-UPDOWN: Interface Dot11Radio0, changed state 
      to up
*Mar  1 00:00:26.352: %LINEPROTO-5-UPDOWN: Line protocol on Interface 
      Dot11Radio0, changed state to up
*Mar  1 00:00:29.440: %LWAPP-5-CHANGED: LWAPP changed state to DISCOVERY
*Mar  1 00:00:29.475: %LINK-5-CHANGED: Interface Dot11Radio0, changed state 
      to reset
*Mar  1 00:00:29.704: %LINK-3-UPDOWN: Interface Dot11Radio1, changed state 
      to up
*Mar  1 00:00:30.121: Logging LWAPP message to 255.255.255.255.

%SYS-6-LOGGINGHOST_STARTSTOP: Logging to host 255.255.255.255 started - CLI 
      initiated
%LINK-3-UPDOWN: Interface Dot11Radio0, changed state to up
%LINK-5-CHANGED: Interface Dot11Radio1, changed state to reset
%LINK-3-UPDOWN: Interface Dot11Radio1, changed state to up
%LINK-5-CHANGED: Interface Dot11Radio0, changed state to reset
%LINK-3-UPDOWN: Interface Dot11Radio0, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface Dot11Radio1, changed state 
      to up
Translating "CISCO-LWAPP-CONTROLLER"...domain server (255.255.255.255)
%DHCP-6-ADDRESS_ASSIGN: Interface FastEthernet0 assigned DHCP address 
      A.B.C.D, mask 255.0.0.0, hostname AP001b.d4e3.a81b
%LWAPP-3-CLIENTEVENTLOG: Controller address 192.168.10.5 obtained through DHCP
%LWAPP-3-CLIENTEVENTLOG: Controller address 192.168.10.5 obtained through DHCP

If you use a Cisco IOS DHCP server, enter the show ip dhcp binding command in order to view the list of the DHCP addresses assigned to DHCP clients. Here is an example:

2800-ISR-TSWEB#show ip dhcp binding

Bindings from all pools not associated with VRF:
IP address          Client-ID/              Lease expiration        Type
                    Hardware address/
                    User name
192.168.25.1        000b.855b.fbd0          Jun 29 2007 11:49 AM    Automatic

On the WLC CLI, you can enter the show ap summary command in order to verify that the APs registered with the WLC. Here is an example:

    ((Cisco Controller) >show ap summary

    AP Name        Slots  AP Model   Ethernet MAC       Location          Port
    -------------  -----  ---------  -----------------  ----------------  ----
    ap:5b:fb:d0    2      AP1010     00:0b:85:5b:fb:d0  default_location  1

If you have Wireless LANs configured, you can enter the show client summary command in order to see the clients that are registered with the WLC:

(Cisco Controller) >show client summary

    Number of Clients................................ 1

    MAC Address        AP Name        Status         WLAN  Auth  Protocol  Port
    -----------------  -------------  -------------  ----  ----  --------  ----
    00:40:96:a1:45:42  ap:64:a3:a0    Associated     4     Yes   802.11a   1

Troubleshoot

Use this section in order to troubleshoot your configuration.

The Output Interpreter Tool (registered customers only) supports certain show commands. Use the Output Interpreter Tool in order to view an analysis of show command output.

Note: Refer to Important Information on Debug Commands before you use debug commands.

Enter the debug dhcp message enable command on the WLC in order to view the sequence of events that occur between the DHCP server and client. Here is an example:

(Cisco Controller) >Thu Jun 28 17:07:53 2007: 00:0b:85:5b:fb:d0 
      dhcp option len, 
    including the magic cookie = 38
Thu Jun 28 17:07:53 2007: 00:0b:85:5b:fb:d0 dhcp option: 
      received DHCP DISCOVER msg
Thu Jun 28 17:07:53 2007: 00:0b:85:5b:fb:d0 dhcp option: 
      skipping option 57, len 2
Thu Jun 28 17:07:53 2007: 00:0b:85:5b:fb:d0 dhcp option: 
      skipping option 55, len 6
Thu Jun 28 17:07:53 2007: 00:0b:85:5b:fb:d0 dhcp option: 
    vendor class id = Airespace.AP1200 (len 16)
Thu Jun 28 17:07:53 2007: 00:0b:85:5b:fb:d0 dhcpParseOptions: options end, 
    len 38, actual 64
Thu Jun 28 17:07:53 2007: dhcpd: sending 300 bytes raw   
    0.0.0.0:68 -> 10.77.244.212:1067
Thu Jun 28 17:07:53 2007: dhcpd: Received 300 byte dhcp packet 
    from 0xd4f44d0a 10.77.244.212:68
Thu Jun 28 17:07:58 2007: 00:0b:85:5b:fb:d0 dhcp option len, including 
    the magic cookie = 50
Thu Jun 28 17:07:58 2007: 00:0b:85:5b:fb:d0 dhcp option: received DHCP 
      REQUEST msg
Thu Jun 28 17:07:58 2007: 00:0b:85:5b:fb:d0 dhcp option: requested ip = 
      192.168.25.1
Thu Jun 28 17:07:58 2007: 00:0b:85:5b:fb:d0 dhcp option: server id = 
      192.168.25.10
Thu Jun 28 17:07:58 2007: 00:0b:85:5b:fb:d0 dhcp option: skipping option 57, 
      len 2
Thu Jun 28 17:07:58 2007: 00:0b:85:5b:fb:d0 dhcp option: skipping option 55, 
      len 6

This is the debug lwapp packet enable command output from the WLC that indicates that DHCP option 43 is used as the discovery method in order to discover WLC IP addresses:

Thu Jun 28 17:51:47 2007: Received LWAPP DISCOVERY REQUEST from AP 
      00:0b:85:5b:fb:d0 
    to 00:0b:85:33:84:a0 on port '1'
Thu Jun 28 17:51:47 2007: Successful transmission of LWAPP Discovery-Response 
    to AP 00:0b:85:5b:fb:d0 on Port 1
Thu Jun 28 19:22:39 2007: Start of Packet
Thu Jun 28 19:22:39 2007: Ethernet Source MAC (LRAD):      00:D0:58:AD:AE:CB
Thu Jun 28 19:22:39 2007: Msg Type       :
Thu Jun 28 19:22:39 2007:    DISCOVERY_REQUEST
Thu Jun 28 19:22:39 2007: Msg Length     :   31
Thu Jun 28 19:22:39 2007: Msg SeqNum     :   0
Thu Jun 28 19:22:39 2007: 
    	IE            :   UNKNOWN IE 58
Thu Jun 28 19:22:39 2007: 	IE Length     :   1
Thu Jun 28 19:22:39 2007: 	Decode routine not available, Printing Hex Dump
Thu Jun 28 19:22:39 2007: 00000000: 03                                                .
Thu Jun 28 19:22:39 2007: 

The value of the IE 58 parameter indicates the discovery type. For DCHP Option 43 it is 3.

If you use the Cisco IOS DHCP server on the router, you can enter the debug dhcp detail command and the debug ip dhcp server events command in order to view the DHCP client and server activity. Here is an example from the debug ip dhcp server events command:

*Jun 28 11:49:33.107: DHCPD: Sending notification of DISCOVER:
*Jun 28 11:49:33.107:   DHCPD: htype 1 chaddr 000b.855b.fbd0
*Jun 28 11:49:33.107:   DHCPD: remote id 020a0000c0a8190a01000000
*Jun 28 11:49:33.107:   DHCPD: circuit id 00000000
*Jun 28 11:49:33.107: DHCPD: Seeing if there is an internally specified 
      pool class:
*Jun 28 11:49:33.107:   DHCPD: htype 1 chaddr 000b.855b.fbd0
*Jun 28 11:49:33.107:   DHCPD: remote id 020a0000c0a8190a01000000
*Jun 28 11:49:33.107:   DHCPD: circuit id 00000000
*Jun 28 11:49:38.603: DHCPD: Sending notification of ASSIGNMENT:
*Jun 28 11:49:38.603:  DHCPD: address 192.168.25.1 mask 255.255.255.0
*Jun 28 11:49:38.603:   DHCPD: htype 1 chaddr 000b.855b.fbd0
*Jun 28 11:49:38.603:   DHCPD: lease time remaining (secs) = 86400
*Jun 28 11:49:38.607: DHCPD: Sending notification of ASSIGNMENT:
*Jun 28 11:49:38.607:  DHCPD: address 192.168.25.1 mask 255.255.255.0
*Jun 28 11:49:38.607:   DHCPD: htype 1 chaddr 000b.855b.fbd0
*Jun 28 11:49:38.607:   DHCPD: lease time remaining (secs) = 86400

Enter the show ip dhcp binding command in order to view the list of the DHCP addresses assigned to DHCP clients.

2800-ISR-TSWEB#show ip dhcp binding

Bindings from all pools not associated with VRF:
IP address          Client-ID/              Lease expiration        Type
                    Hardware address/
                    User name
192.168.25.1        000b.855b.fbd0          Jun 29 2007 11:49 AM    Automatic 

Related Information

• Upgrading Autonomous Cisco Aironet Access Points to Lightweight Mode
• How to configure the Lightweight Access Point in order to join the respective Wireless LAN Controller
• Wireless Product Support