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    DHCP Duplicate Client Differentiation Using Client Subinterface Overview

    In some network environments, client IDs and MAC addresses might not be unique, resulting in duplicate clients. For example, two network adapters might be manufactured with the same hardware address, resulting in a duplicate MAC address among the DHCP clients attached to the router. A duplicate DHCP client occurs when a client attempts to get a lease, and that client has the same client ID or the same MAC address as an existing DHCP client.

    When an extended DHCP local server receives a request from a new client that has a duplicate ID or MAC address, the DHCP server terminates the address lease for the existing client and returns the address to its original address pool. The DHCP server then assigns a new address and lease to the new client.

    By default, a DHCP local server uses the subnet information to differentiate between duplicate clients. However, in some cases, this level of differentiation is not adequate. For example, when multiple subinterfaces share the same underlying loopback interface with the same preferred source address, the interfaces appear to be on the same subnet. In this situation, the default configuration prevents duplicate clients.

    You can provide greater differentiation between duplicate clients by configuring DHCP to consider the client subinterface when duplicate clients occur. In this optional configuration, DHCP uniquely identifies:

    • The subnet on which the client resides
    • The subinterface on which the client resides
    • The client within the subnet

    Modified: 2017-08-31