DHCP option 82 assigns IP addresses depending on the switch port to which the device is connected. The switch in question uses DHCP option 82 to supplement the DHCP request with the Circuit ID, which identifies the port that this device is connected to. This information is then used by the DHCP server to assign a specific IP address. This establishes a relationship between an IP address and a location, which simplifies network monitoring.
In LANconfig under , you can set the switch-port based assignment of IP addresses for each port separately by clicking on Relay info.
After selecting the switch port that is automatically added by the DHCP option 82, you can set the following:
- Circuit-ID
- This is the storage location for the "Circuit ID" used for address assignment and inserted by the relay agent or switch using DHCP option 82. The string is evaluated case-sensitive. Depending on the particular switch, the "Circuit ID" is delivered by the relay agent in various formats and stored accordingly. This can be a complete hexadecimal string with leading 0x. An alternative syntax allows the entry of binary values, as with the user class identifier or vendor class identifier:
Binary values are specified in the form {value/bit length}. The value can be specified as decimal, hexadecimal (leading 0x) or octal (leading 0), and the available bit lengths are 8, 16, 24, 32, 48 and 64. The value is stored in big-endian representation. Little-endian representation requires "negative" bit lengths: -8, -16, -24, -32, -48 or -64
A circuit ID (00 02 00 1e 4d 45 53 2d 33 37 32 38) can be stored in one of the following representations:
- 0x0002001e4d45532d33373238
- {0/8} {2/8} {30/16}
- {0x00/8} {0x02/8} {0x1e/16}
- {00/8} {02/8} {036/16}
- IP address
- Enter the IP address assigned to the host on this port. Do not leave this column unspecified (0.0.0.0). Otherwise only one host per circuit ID would be able to authenticate. As long as there is an entry in the DHCP table, any DHCP messages from other hosts using the same circuit ID would be ignored. In other words, if you want to operate another host on the port, the previous one must either log off correctly (e.g. under Microsoft Windows: ipconfig/release) or the entry must be deleted from the DHCP table.
- Station name
- Enter the name that is to be used to identify the station. If the station does not communicate its name, the device will use the name entered here.
- Image alias
- If the client uses the BOOTP protocol, you can select a boot image that the client should use to load its operating system from.
Note: Enter the server providing the boot image and the name of the file on the server in the boot image table.
