DHCP Option 125 is a vendor-specific feature that in our case is required if you are using AIM’s multi-subnet mode. It provides additional information to the Infinity Receivers and Transmitters when requesting an IP Address from the DHCP Server. The option gives the endpoints the IP Address of the Primary and any other Backup or Satellite AIM Servers that reside on the network. It is also used to setup Satellite AIM’s. To simplify the process you can download a Windows DHCP Option 125 application, if you require.
New/factory reset endpoints only read the first IP address in the Option 125 list until it has communicated with the Primary AIM and security keys have been exchanged. Thereafter, it will contact any of the IP addresses listed in the event of an AIM failure.
The information is provided in a hex code which can be formatted slightly differently depending on the DHCP Server, but will typically look like:-
The Hex code is broken down to the following elements:
|0000621f||Adder Enterprise number (25119) – fixed|
|01||Sub-option 1 – fixed|
|0a001405||10.0.20.5 (Primary AIM)|
|0a001406||10.0.20.6 (Backup AIM)|
The Adder Enterprise and Sub-option 1 elements of the hex code are fixed and do not change, however the IP Address and Length A & B hex values will change depending on your own Infinity installation.
The first IP address in the list must always be the Primary AIM Server, followed by the Backup then Satellite AIM’s.
There are a number of online websites that help you convert an IP Address into a hex number. Alternatively, you could use a Calculator with a Hex and Dec function which you may find is built into your computers operating system.
An IP Address hex code can be calculated by converting each of the four decimal octet’s individually into hex then combining them.
10.0.20.5 = 0a001405
The Length A and B values will change depending on the number of IP Addresses that are included.
Each hex IP Address is 4-bytes in length, for example:
|1||1||1||1 (Total 4 bytes)|
The Length B value is the total byte count of the hex IP Addresses. In the example below, there are 2x hex IP addresses. Each hex IP address is 4-bytes, therefore Length B is 8-bytes in total which equates to 08 in hex.
Length A includes Sub-option 1 (1-byte) and the Length B value (1-byte) plus the total of Length B (8-bytes). This makes the total 10-bytes which equates to 0a in hex.
In the following example an additional IP address will be added to the existing DHCP option 125 hex code shown previously. It will be a Satellite AIM whose IP address is 10.0.30.1 which is on a different subnet.
Step 1: Convert IP Address 10.0.30.1 into hex, which is 0a001e01. Add it to the end of the hex code.
Step 2: Calculate Length B. Since we now have 3x IP addresses and each hex IP address is 4-bytes, the total length is 12-bytes which equates to 0c in hex.
Step 3: Calculate Length A. This is the total of Length B plus the byte count for Sub-option 1 and the Length B value. The Sub-option 1 and Length B value are 1-byte each, the Length B total is 12-bytes. This makes the total of Length A to be 14-bytes which equate to 0e in hex.
Example: ISC DHCPd, Adder, AIM servers: 10.0.20.5, 10.0.20.6
set vendor-string = option vendor-class-identifier;
option space adder code width 1 length width 1; option adder.aim-servers code 1 = array of ip-address;
option space vivso code width 4 length width 1; option vivso.iana code 0 = string;
option vivso.adder code 25119 = encapsulate adder;
option option-125 code 125 = encapsulate vivso;
default-lease-time 3600; max-lease-time 7200;
match if option vendor-class-identifier = "adder";
subnet 10.0.20.0 netmask 255.255.255.0
allow members of "adder"; vendor-option-space adder;
option domain-name "customer.com"; option domain-name-servers 10.0.20.2; option routers 10.0.20.1;
option vivso.iana 01:01:01;
option adder.aim-servers 10.0.20.5,10.0.20.6; range 10.0.20.100 10.0.20.200;
Example: Cisco IOS DHCPd, Adder, AIM servers: 10.0.20.5, 10.0.20.6
network 10.0.20.0 255.255.255.0 default-router 10.0.20.1 option 60 ascii "adder"
option 125 hex 0000.621f.0a01.080a.0014.050a.0014.06
Example: HP 2920 Switch. Adder AIM Servers: 10.0.10.10, 10.0.10.11
network 10.0.20.0 255.255.255.0
option 125 hex "0000621f0a01080a000a0a0a000a0b"
range 10.0.20.10 10.0.20.250
Example: Setting up DHCP on Microsoft Windows Server 2012
- Open the DHCP server configuration console (found within administration tools).
- Expand the server tree in question on the left hand side.
- Right click the IPv4 object in the left hand tree Select "Set Predefined Options".
- With the Option Class set to "DHCP Standard Options" add in a new option 125 and label accordingly.
- Within the DHCP server option, enable this option and add the relevant details accordingly, e.g. 0000621f0a0108C0A80114C0A80115 (with the Hex IP addresses updated)
- For our setup to take immediate effect a restart the DHCP service was required.
When a transmitter or receiver has successfully acquired a DHCP IP address along with Option 125, it will automatically assume that it is under the AIM Managers control and you should see the IP address of the AIM Manager(s) listed on the About page.
For example, the screenshot below has been taken from ALIF2020. In the AIM section that you will see that the AIM servers are 10.0.20.5 and 10.0.20.6 respectively.