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[SRX\EX] How to configure the 'DHCP option 43'

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Knowledge Base ID: KB23834
Last Updated: 04 Sep 2013
Version: 3.0

Summary:
This article provides information on how to configure the DHCP option 43 on the EX\SRX platform.

Problem or Goal:
  • The DHCP Option 43 feature helps the Access Point (AP) to associate with the WLC (Wireless controller) in a L3 environment (AP in one network and WLC in one network).

  • The Access Point 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 43 TLV option. The TLV should contain the IP address of the WLC, with which the AP needs to be associated.

Cause:

Solution:
EX\SRX configuration:

The following command has to be issued to configure DHCP Option 43 on EX-series switches or SRX-Firewall:
Switch@juniper#set system services dhcp option 43 byte-stream "105 112 58 49 48 46 50 53 52 46 50 51 55 46 49"
("105 112 58 49 48 46 50 53 52 46 50 51 55 46 49" is same as "ip:10.254.237.1" in ASCII code)
Topology:
EX----AP (V100) IP:10.41.15.100

|

|

WLC (V200) IP: 10.254.237.1
Note: The 'ip:10.254.237.1' string is only for the Trapeze platform. for Cisco APs, you need to use the 'Type' value in the byte stream.

Convert to ASCII:

IP address:10.254.237.1 > 105 112 58 49 48 46 50 53 52 46 50 51 55 46 49
Switch@juniper>show system services dhcp option 43 byte-stream "105 112 58 49 48 46 50 53 52 46 50 51 55 46 49"
The following command can be issued to verify the configuration:
Switch@juniper>show system services dhcp

router {

    10.41.15.1;

}

option 43 byte-stream "105 112 58 49 48 46 50 53 52 46 50 51 55 46 49";

pool 10.41.15.0/24 {

    address-range low 10.41.15.100 high 10.41.15.199;

}
When the AP is connected to the ge-0/0/0 port on v100, it receives an IP address from the EX switch (DHCP server) and immediately synchronizes with the Wireless Controller and forms the association.

Find the AP association status on WLC:
MXR-2# sh ap unconfigured

Serial-Id 1172500006 Model MP-371B IP Address: 10.41.15.100

Port: 1 Vlan: v200 Reason: No Configuration
Trace collected during the association:
MXR-2# sh log trace

SYS Feb 25 03:48:09.208522 NOTICE User cleared trace buffer

APM_MGR Feb 25 03:48:57.179286 DEBUG dap_mgr_handle_delayed_events:TMR_RETRY:SANITY:TMR: restart tmr:

APM_FMX Feb 25 03:49:07.077450 NOTICE AP_TIMER: dap_handle_discovery: Received unknown dap tlv 3

APM_FMX Feb 25 03:49:07.078208 NOTICE AP_TIMER: dap_handle_discovery: Received unknown dap tlv 4

APM_FMX Feb 25 03:49:07.079238 DEBUG dap_handle_discovery(2145) sid 1172500006: AP requests TAPA2,...MX will talk TAPA2

APM_FMX Feb 25 03:49:07.080685 DEBUG dap_process_l2l3_fmx_request(1699) sid 1172500006: ap 1172500006 mesh
0 cap_back: 0 require lic: 0
How to convert the ip:10.254.237.1 string to ASCII:

10.254.237.1 > 105 112 58 49 48 46 50 53 52 46 50 51 55 46 49

Refer to the following link to convert the decimal value to ASCII:

http://www.asciitable.com/

Find the ASCII value for string ip:10.254.237.1:


i = 105

p = 112


: = 58


1 = 49

0 = 48


. = 46


2 =50

5 =53

4 =52


. = 46


2 =50

3 =51

7 =55


. = 46


1 =49


Sample configuration to add option 43 for Cisco LWAP:

The following command is issued to configure option 43 on an EX switch, when the Access Point is a Cisco device:
Switch@juniper#set system services dhcp option 43 byte-stream “hexadecimal string”
The hexadecimal string is assembled as a sequence of TLV values for the Option 43 sub option: Type + Length + Value. The Type is always (for Cisco aironet series) the 0xf1suboption code. The Length is the number of controller management IP addresses multiplied by 4 in Hex. The Value is the IP address of the controller that is sequentially listed in Hex.

For example:

  • A controller for the network is present with the 10.254.237.1 management interface IP address.

  • The type is 0xf1.

  • The length is 1 * 4 = 4 = 0x04.

  • The IP addresses translate to 0xac 0x10 0x15 0x8c.

  • When the string is assembled, it yields f104ac10158c.

The following command is added to the DHCP scope:
Switch@juniper#set system services dhcp option 43 byte-stream “0xf1 0x04 0xac 0x10 0x15 0x8c”

Purpose:
Configuration
Implementation
Specifications

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