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DHCPv4 Duplicate Client Management

DHCPv4 Duplicate Client In Subnet Overview

In some network environments, client IDs and hardware addresses (MAC addresses) might not be unique, resulting in duplicate clients. A duplicate DHCP client occurs when a client attempts to get a lease, and that client has the same client ID or the same hardware address as an existing DHCP client—the existing client and the new client cannot exist simultaneously, unless you have configured the optional duplicate client support.

By default, DHCP local server and DHCP relay agent use 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.

You can enable support for duplicate clients in a subnet by configuring DHCP to use additional information to uniquely identify clients—the additional information is either the client incoming interface or the option 82 information in the DHCP packets. Using the option 82 information provides the following important benefits:

  • You can configure DHCP relay to preserve and use the remotely created option 82.

  • DHCP local server can support an environment in which an aggregation device is present between the client and the DHCP server.

When configured to support duplicate clients in the subnet, DHCP uses the following information to distinguish between the duplicate clients:

  • The subnet on which the client resides

  • The client ID or hardware address

  • The duplicate clients option you configure—either the client incoming interface or the option 82 information in the client’s incoming DHCP packets

    Note:

    Starting in Junos OS Release 16.1R5, 16.2R2, 17.1R2, and 17.2R1, only the ACI (suboption 1) and ARI (suboption 2) values from the option 82 information are used. Other suboptions, such as Vendor-Specific (suboption 9), are ignored.

Guidelines for Configuring Support for DHCPv4 Duplicate Clients

When configuring DHCPv4 duplicate client support, consider the following guidelines:

  • If you want to preserve the remotely-created option 82 information, use the option 82 option with the duplicate-clients-in-subnet statement to distinguish between duplicate clients. If there is no remotely created option 82 in the incoming DHCP packets, the router locally creates the option 82 information.

  • If you want to use the locally-created option-82, use the incoming-interface option with the duplicate-clients-in-subnet statement to distinguish between duplicate clients.

  • Only the ACI (suboption 1) and ARI (suboption 2) values from the option 82 information are used. Other suboptions, such as Vendor-Specific (suboption 9) are ignored.

  • DHCP relay agent and DHCP local server in the same routing instance must have the same the duplicate-clients-in-subnet configuration.

  • For the Layer 3 wholesale model:

    • The wholesaler and retailer logical system/routing instances must have the same duplicate-clients-in-subnet statement configuration.

    • For DHCP relay, the wholesaler and the retailer routing contexts must both have the relay-option-82 statement configured with the Agent Circuit ID suboption (suboption 1) in option 82.

Configuring the Router to Distinguish Between DHCPv4 Duplicate Clients Based on Option 82 Information

Duplicate clients occur when two clients in a subnet have the same hardware address or the same client ID.

The following two procedures describe how to configure the router to use the option 82 information in the incoming packets to differentiate between duplicate clients. The first procedure describes the configuration for DHCP relay agent The second procedure is for DHCP local server.

Note:

Only the ACI (suboption 1) and ARI (suboption 2) values from the option 82 information are used. Other suboptions, such as Vendor-Specific (suboption 9) are ignored.

To configure the DHCP relay agent to differentiate between duplicate clients based on option 82 information:

  1. Specify that you want to configure DHCP relay agent.
  2. Configure DHCP relay to insert option 82 information if there is no remotely created option 82. Use the default setting, which inserts the interface ID rather than the optional interface description.
  3. Configure the router to always accept DHCP client packets that contain option 82 information.
    Note:

    The trust-option-82 statement must always be enabled so the router can process incoming DHCP client packets that contain option 82 information when the packets have a gateway IP address (giaddr) of 0 (zero).

  4. Configure DHCP relay to use the remotely created option 82 information to distinguish between duplicate clients. If there is no remotely created option 82 in the traffic, the router locally creates the option 82 information.
    Note:

    Make sure that the always-write-option-82 statement is not enabled, as the statement will overwrite the remotely created option 82.

To configure the DHCP local server to differentiate between duplicate clients based on the option 82 information:

  1. Specify that you want to configure DHCP local server.

  2. Configure the duplicate client support with the option-82 option.

Configuring the Router to Distinguish Between DHCPv4 Duplicate Clients Based on Their Incoming Interfaces

Duplicate clients occur when two clients in a subnet have the same hardware address or the same client ID.

The following two procedures describe how to configure the router to use the clients’ incoming interface to differentiate between duplicate clients. The first procedure describes the configuration for DHCP relay agent: the second procedure is for DHCP local server.

To configure the DHCP relay agent to differentiate between duplicate clients based on the client incoming interface:

  1. Specify that you want to configure DHCP relay agent.
  2. Configure the duplicate client support with the incoming-interface option.
  3. Configure DHCP relay to insert option 82 information if the information is not specified remotely. Use the default setting, which inserts the interface ID rather than the optional interface description.
    Note:

    Only the ACI (suboption 1) and ARI (suboption 2) values from the option 82 information are used. Other suboptions, such as Vendor-Specific (suboption 9) are ignored.

  4. Configure the router to overwrite any remotely supplied option 82 information in incoming packets.
  5. Configure the router to always accept DHCP client packets that contain option 82 information.
    Note:

    The trust-option-82 statement must always be enabled so the router can process incoming DHCP client packets that contain option 82 information when the packets have a gateway IP address (giaddr) of 0 (zero).

To configure the DHCP local server to differentiate between duplicate clients based on the client incoming interface:

  1. Specify that you want to configure DHCP local server.

  2. Configure the duplicate client support with the incoming-interface option.

Related Documentation

Change History Table

Feature support is determined by the platform and release you are using. Use Feature Explorer to determine if a feature is supported on your platform.

Release
Description
16.1R5
Starting in Junos OS Release 16.1R5, 16.2R2, 17.1R2, and 17.2R1, only the ACI (suboption 1) and ARI (suboption 2) values from the option 82 information are used.