What is DHCP Option 43? Have you learned it?

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When the AP and AC are in the same network segment, the AP will actively send broadcast messages and can directly find the AC in the network segment. When the AP and AC are in different network segments, broadcast messages cannot be transmitted across network segments. The AP can only find the AC and go online by specifying the AC IP address on the AP.

If there are only a few APs, you can use a simple, manual AC assignment method, that is, log in to the AP and type the command to assign the AC on the AP. If there are many APs, the manual AC assignment method is not applicable because of duplication of operations and high workload. You have to use the batch assignment method, that is, specify the AC IP address through the DHCP option 43 field to implement batch AP online.

option 43

Option 43 is a custom option used to indicate the AC IP address. The AP obtains the AC IP address information through the option 43 field of DHCP, and then sends a unicast message to find the corresponding AC to achieve zero-configuration AP online. The APs mentioned here are all FIT APs.

Option 43 has a prerequisite: the AP and the DHCP server must be reachable to each other, ensuring that the AP can normally exchange messages with the DHCP server; the AP and the AC must be reachable to each other, ensuring that the AP can normally exchange messages with the AC.

Option 43 format

There are three ways to express the value of option 43: hexadecimal, decimal, and ASCII. For example, when the AC IP address is 192.168.100.1, the value is c0a86401 in hexadecimal, 192.168.100.1 in decimal, and 3139322e3136382e3130302e31 in ASCII, where "." corresponds to 2e. The most common format is the hexadecimal value, which is also the most compatible.

Sometimes, it is used together with the option 60 field, which indicates the device manufacturer name.

Huawei configuration option 43

Taking Huawei devices as an example, configure option 43 and the AC IP address is 192.168.100.1.

dhcp server option 43 hex 0104c0a86401.

Among them, 01 indicates type 1. Huawei has three types, namely hexadecimal, decimal, and ASCII. 04 indicates the field length. Each IP address occupies 4 bytes. The calculation formula is the number of IP addresses multiplied by 4. c0a86401 is the hexadecimal number of 192.168.100.1.

H3C Option 43 Configuration

Taking H3C equipment as an example, configure option 43 and the AC IP address is 192.168.100.1.

[AC]dhcp server ip-pool vlan10[AC-dhcp-pool-vlan100] network 192.168.10.0 mask 255.255.255.0[AC-dhcp-pool-vlan100] gateway-list 192.168.10.1[AC-dhcp-pool-vlan100] option 43 hex 80070000 01 C0A86401.

Among them, 80​ indicates the type, which is a fixed value. 07​ indicates the length of the following content, which means there are 7 hexadecimal numbers and an AC IP address. If the value is 0B, it means there are 11 hexadecimal numbers, that is, two AC IP addresses. 0000​ indicates the service type, which is a fixed value. 01​ indicates the number of IP addresses. c0a86401 is the hexadecimal number of 192.168.100.1.

Configuring Option 43 on Cisco

Taking Cisco equipment as an example, configure option 43 and the AC IP address is 192.168.100.1.

Cisco(config)#ip dhcp pool APCisco(dhcp-config)#network 192.168.10.0 /24Cisco(dhcp-config)#default-route 102.168.10.1Cisco(dhcp-config)#option 43 hex f1040c0a86401.

Among them, f1 indicates the type, which is a fixed value. 04 indicates the length of the following content, which is 4 hexadecimal numbers, and there is an AC IP address. c0a86401 is the hexadecimal number of 192.168.100.1.

Linux ISC DHCP server configuration option 43

Taking the Linux ISC DHCP server as an example, configure option 43 and option 60. The Cisco AC IP address is 192.168.247.5, and the Huawei AC IP address is 192.168.247.55.

Configuration file path

/etc/dhcp/dhcpd.conf

 #ddns - update - style interim ; option space Cisco_LWAPP_AP ; option Cisco_LWAPP_AP .server - address code 241 = array of ip - address ; option space Huawei - AP6050DN ; option Huawei - AP6050DN .server - address code 1 = array of ip - address ;
 subnet 192.3 . 1.0 netmask 255.255 . 255.0 { authoritative ; range 192.3 . 1.100 192.3 . 1.254 ; option subnet - mask 255.255 . 255.0 ; option broadcast - address 192.3 . 1.255 ; option routers 192.3 . 1.1 ; option domain - name "huawei.com" ; option domain - name - servers 192.168 . 247.2 , 192.168 . 247.3 ; default - lease - time 300 ;
 class "Cisco-AP-c1700" { match if option vendor - class - identifier = "Cisco-AP-c1700" ; option vendor - class - identifier "Cisco-AP-c1700" ; vendor - option - space Cisco_LWAPP_AP ; option Cisco_LWAPP_AP .server - address 192.168 . 247.5 ; }
 class "huawei AP" { match if option vendor - class - identifier = "huawei AP" ; option vendor - class - identifier "huawei AP" ; vendor - option - space Huawei - AP6050DN ; option Huawei - AP6050DN .server - address 192.168 . 247.55 ; }
 subnet 192.168 . 1.0 netmask 255.255 . 255.0 { } #

Windows DHCP server configuration option 43

Take Windows DHCP server as an example, configure option 43, and the AC IP address is 192.168.22.1.

Enter the Window DHCP server configuration interface

Write hexadecimal numbers in "binary"

Among them, 03 indicates type 3, 0C indicates the length of the following content, which means there are 12 hexadecimal numbers behind. 192.168.22.1 is regarded as a string and converted into hexadecimal. 31 39 32 2E 31 36 38 2E 32 32 2E 31 is the ASCII code corresponding to the IP address 192.168.22.1. 0-9 corresponds to 30-39 respectively, and "." corresponds to 2E. "ASCII" does not need to be filled in, it is automatically generated according to the "Binary" content.

Infoblox DHCP server configuration option 43

Take the Infoblox server as an example, configure option 43, and the AC IP address is 10.6.2.1.

The IP address 10.6.2.1 is converted into the format of 03:08:31:30:2e:36:2e:32:2e:31. The two characters need to be separated by a colon.

Among them, 03 indicates type 3, and 08 indicates the length of the following content, which means there are 8 hexadecimal numbers following it. 10.6.2.1 is regarded as a string and converted into hexadecimal. 31:30:2e:36:2e:32:2e:31 is the ASCII code corresponding to the IP address 10.6.2.1.

at last

The above option 43 configuration is only used as an example. In actual use, you need to check the product descriptions of the DHCP server and AP respectively, and use matching formats or types to ensure that the AP can recognize the content of option 43. For example: when configuring an Aruba AP, you can directly fill in the AC IP address on Infoblox without converting the format.