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> # Configuring DHCP External Server to Uniquely Identify Clients with Duplicate MAC Addresses

> You can configure the DHCP external server application to use a combination of the MAC address and giaddr to uniquely identify DHCP clients attached to the router. This behavior is also referred to as duplicate MAC mode. By default, DHCP external server uses only the MAC address to uniquely identify DHCP clients. The default behavior is also referred to as unique MAC mode.

> Enabling duplicate MAC mode is useful if you are using the DHCP external server application to manage DHCP clients from multiple DHCP relays in network environments where the same MAC address might be assigned to more than one client. In such environments, DHCP external server must use a combination of the MAC address and giaddr to uniquely identify the DHCP clients it manages.

> Preserving the client’s dynamic subscriber interface is useful if deleting and re-creating the dynamic subscriber interface might result in a service interruption.

> To configure the DHCP external server application to use a combination of the MAC address and giaddr to uniquely identify DHCP clients, also known as enabling duplicate MAC mode:

> - Issue the dhcp-external duplicate-mac-address command from Global Configuration mode:
> `
> host1:vr1(config)#dhcp-external duplicate-mac-address
> `

> To restore the DHCP external server default behavior to use only the MAC address to uniquely identify DHCP clients, also known as enabling unique MAC mode:

> - Issue the no dhcp-external duplicate-mac-address command from Global Configuration mode:
> `
> host1:vr1(config)#no dhcp-external duplicate-mac-address
> `

> | Note: Unlike other commands for configuring DHCP external server, the dhcp-external duplicate-mac-address command applies globally to all instances of the DHCP external server application on the router, and is not issued on a per-VR basis.

> #### Updated: 2009-04-20

> Copyright © 2009, Juniper Networks, Inc. All rights reserved. Trademark Notice.