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DHCP server has the ability to include DHCP codes and options. Voice over IP devices, such as IP phones need to be able to retrieve their configuration name files from a Management VOICE server on the network and sometimes is required that more than one TFTP server is to be used by the clients. The client,  in this case the Cisco Phone, will send a request with option 150 to the DHCP server to obtain the needed information.

The format of the option is:
Code Len IPv4 Configuration Server Address(es)
| 150 | n | IPv4 address | ...
For more information refer to the RFC5859

In the FortiOS Option codes are represented in a option value/HEX value pairs.

To have a list of TFTP Server Addresses, both IP addresses will need to be appended in a hexadecimal notation.

For example:

Server1:  = IP Hex: C0A80101
Server2: = IP Hex: C0A80102
config system dhcp server
edit 3
set default-gateway
set interface "port1"
config ip-range
edit 1
set end-ip
set start-ip
set netmask
set option1 150 'C0A80101C0A80102'
The option contains both IP addresses appended in hexadecimal notation.