Tag: DHCP

Configure Cisco IOS DHCP to use vendor class IDs

The IOS DHCP server can be configured to provide different address information to clients based on information they provide via DHCP option 60.

DHCP Option 60 is the “vendor class identifier option” that allows the DHCP client to identify its type so that custom configuration can be applied.

Configuring the DHCP Client

For custom address configuration to be applied the client must specify option 60. This is configured with the “ip dhcp client class-id XXX” command, where XXX is an ASCII label to use. For example:

interface Vlan10
  description ** Corporate LAN - Management Address **
  ip dhcp client class-id CUSTOM_CLASS
  ip address dhcp
  end

Configuring the DHCP Server

To configure the IOS DHCP server you must specify a default class and then a class that will match against DHCP option 60. When matching against option 60 you must convert the ASCII string the client sends (e.g. “CUSTOM_CLASS”) to hexadecimal.

ip dhcp class DEFAULT
  remark IP addresses for devices not providing a class-id
!
ip dhcp class CUSTOM_CLASS
  remark IP addresses for devices providing "CUSTOM_CLASS"
  option 60 hex 435553544f4d5f434c415353

With the matching setup the DHCP pool configuration can be split into the custom class and a default:

ip dhcp pool LAN
  network 192.168.0.0 255.255.255.0
  default-router 192.168.0.254
  class CUSTOM_CLASS
    address range 192.168.0.210 192.168.0.220
  class DEFAULT
    address range 192.168.0.0 192.168.0.200

If this doesn’t work the following debug commands may be helpful in identifying the cause of the problem:

debug ip dhcp server class
debug ip dhcp server packet detail