client-identifier command

Router(dhcp-config)#client-identifier unique-identifier

Specifies the unique identifier for DHCP clients. This command is used for DHCP requests. DHCP clients require client identifiers. The unique identification of the client is specified in dotted hexadecimal notation, for example, 01b7.0813.8811.66, where 01 represents the Ethernet media type. Every request sent from a DHCP client to DHCP servers contains a hardware type and a client hardware address. For Ethernet and 802.11 wireless clients, the hardware type is always 01. The client hardware address is simply the MAC address of the client's network (Ethernet or Wireless) interface. Every request sent from a DHCP client to DHCP servers may optionally also contain a DHCP Client Identifier option. This is an arbitrary value that may be used to identify a client instead of the hardware type and client hardware address. Traditionally, the DHCP Client Identifier (when present) has been set to a value equal to the hardware type followed by the client hardware address. (For example, 01 00 01 02 a0 bc d3.)

Example:
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#ip dhcp pool AccessPoint1
Router(config)#client identifier 01ab.cdef.ghij.kl
Router(config)#client-name AccessPoint1

References:
http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfdhcp.html#wp1001108