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Using Lightweight DHCPv6 Relay Agent (LDRA)

 

In Layer 2 networks that have many nodes on a single link, a DHCP server would normally be unaware of how a DHCP client is attached to the network. In a DHCPv6 deployment, you can use a Lightweight DHCPv6 Relay Agent (LDRA) to add relay agent information to a DHCPv6 message to identify the client-facing interface of the access node that received the message. The server can use this information to assign IP addresses, prefixes, and other configuration parameters for the client.

DHCPv6 relay agents are typically used to forward DHCPv6 messages between clients and servers or other relay agents when they are not on the same IPv6 link node. The relay agent can add information to the messages before relaying them. When the client and server reside on the same IPv6 link, LDRA enables a switching device to perform the function of intercepting DHCPv6 messages and inserting relay agent information that can be used for client identification. The LDRA acts as a relay agent, but without performing the routing function necessary to forward messages to a server or relay agent that resides on a different IPv6 link.

When the LDRA receives a DHCPv6 Solicit message from a client, it encapsulates that message within a DHCPv6 Relay-Forward message, which it then forwards to the server or another relay agent. Before it forwards the Relay-Forward message, the LDRA can also insert relay information by using one or more of the following options:

  • option-16 (Vendor ID)—Option 16 provides the server with information about the vendor that manufactured the hardware on which the DHCPv6 client is running. Option 16 is the DHCPv6 equivalent of the vendor-id suboption of DHCP option 82.

  • option-18 (Interface ID)—A unique identifier for the interface on which the client DHCPv6 packet is received. Suboptions can be configured to include a prefix with the interface ID or to change the type of information used to identify the interface. Option 18 is the DHCPv6 equivalent of the circuit-id suboption of DHCP option 82.

  • option-37 (Remote ID)—A unique identifier for the remote host. Suboptions can be configured to include a prefix with the remote ID or to change the interface portion of the ID. Option 37 is the DHCPv6 equivalent of the remote-id suboption of DHCP option 82.

You must configure LDRA if you configure DHCPv6 options at the [edit vlan vlan-name forwarding-options dhcp-security dhcpv6-options] hierarchy level. Option 16, option 37, and option 79 are included in the Relay-Forward message only if they are explicitly configured. Option 18 is mandatory in Relay-Forward messages and is included even if it is not explicitly configured. However, suboptions of option 18 are included only if they are configured using the option-18 statement at the [edit vlan vlan-name forwarding-options dhcp-security dhcpv6-options] hierarchy level.

To configure LDRA to enable DHCPv6 options:

  1. Configure the switch as an LDRA.
    [edit vlans vlan-name forwarding-options dhcp-security]

    user@switch# set light-weight-dhcpv6-relay
  2. Configure the switch to insert DHCPv6 options in the Relay-Forward message to provide additional information about the client to the server or to another relay agent.
    • To insert option 16:

      [edit vlans vlan-name forwarding-options dhcp-security]

      user@switch# set dhcpv6-options option-16
    • To insert option 18:

      [edit vlans vlan-name forwarding-options dhcp-security]

      user@switch# set dhcpv6-options option-18
    • To insert option 37:

      [edit vlans vlan-name forwarding-options dhcp-security]

      user@switch# set dhcpv6-options option-37
  3. (Optional) Configure a prefix to include additional information with DHCPv6 option 18 or DHCPv6 option 37. For example, to configure a prefix for option 37 to include the switch’s hostname:
    [edit vlans vlan-name forwarding-options dhcp-security dhcpv6-options]

    user@switch# set option-37 prefix host-name


  4. (Optional) Change the type of information used to identify the interface. For example, to specify that option 18 contain the interface description for the logical unit rather than the interface name (the default):
    [edit vlans vlan-name forwarding-options dhcp-security dhcpv6-options]

    user@switch# set option-18 use-interface-description logical
    Note

    To use the interface description rather than the interface name for identifying the interface, the interface description must be specified under interface unit (set interfaces ge-0/0/0 unit 0 description description). If you do not do this, then the interface name is used by default.



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