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FAQ-how to configure sub-option when USG6600 works as DHCP Server for AP

Publication Date:  2019-07-13 Views:  289 Downloads:  0

Issue Description

In the scenario when there are two ACs working in N+1 backup mode, if an AP and the ACs are located in different network segments, the AP cannot discover the ACs through broadcast after it obtains an IP address from the DHCP server. To address this problem, configure Option 43 on the DHCP server to advertise AC IP addresses to the AP.


After Option 43 is configured, the AP unicasts Discovery Request packets to the IP addresses carried in Option 43. If the IP addresses specified by Option 43 do not respond, the AP broadcasts Discovery Request packets to request IP addresses of the ACs in the local network segment.


Usually, an independent device is used as a DHCP server. Perform correct configurations on the selected DHCP server.

In Huawei Router, the IP of AC can be identified in this way:

ip pool ip-pool-name

option 43 sub-option 2 ip-address ip-address &<1-8>

The Option 43 field is set to the IP addresses of the active AC and standby AC

But in USG6600, until the latest version in V100R001C30SPC300, there is no sub-option parameter to be configured. In this case, if USG6600 works as DHCP Server for AP, how can the ap know where AC is?

Here is the topology:

Here is the configuration in USG6600:

[USG6650]dhcp server ip-pool AP                                         

[USG6650-dhcp-ap]option ?                                               

  INTEGER<2-254>  DHCP option code (excluding 3,6,15,44,46,50-54,57-59

[USG6650-dhcp-ap]option 43 ?                                             

  ascii       ASCII string                                                     

  hex         Hexadecimal string                                               

  ip-address  IP addresses 

As we can see from above, there is no sub-option parameter.


In the software version V100R001 of USG6600:

Here is an example :

<sysname> system-view
[sysname] interface GigabitEthernet 1/0/1
[sysname-GigabitEthernet1/0/1] ip address 24
[sysname-GigabitEthernet1/0/1] dhcp select interface

[sysname-GigabitEthernet1/0/1] dhcp server option 43 hex 03 0D 31 39 32 2E 31 36 38 2E 31 30 30 2E 31

In the above example, the first number “03” indicates “sub-option 03”, and the number”0D” indicates the length of IP string after “0D”. “31 39 32 2E 31 36 38 2E 31 30 30 2E 31” indicates the IP of AC is “”

If there are more than on AC, for example, one AC is, and the other is there is acomma (,)  between two IPs.

[sysname-GigabitEthernet1/0/1] dhcp server option 43 hex 03 1B 31 39 32 2E 31 36 38 2E 31 30 30 2E 31 2C 31 39 32 2E 31 36 38 2E 31 30 30 2E 32


In the software version V500R001 of USG6600:

 the parameter “sub-option ” is supported: