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Using DHCPv6 Prefix Delegation to Provide IPv6 Addresses on the Subscriber LAN

You can use Dynamic Host Configuration Protocol for IPv6 (DHCPv6) prefix delegation to automate the delegation of IPv6 prefixes to the customer premises equipment (CPE). With prefix delegation, a delegating broadband network gateway (BNG) router delegates IPv6 prefixes to a CPE router. The requesting router then uses the prefixes to assign global IP addresses to the devices on the subscriber LAN. The requesting router can also assign subnet addresses to subnets on the LAN.

DHCPv6 prefix delegation is useful when the delegating router does not have information about the topology of the networks in which the requesting router is located. In such cases, the delegating router requires only the identity of the requesting router to choose a prefix for delegation.

DHCPv6 prefix delegation replaces the need for Network Address Translation (NAT) in an IPv6 network.

Figure 1 shows how DHCPv6 prefix delegation is used in a dual-stack network.

Figure 1: Delegated Addressing in a Dual-Stack Network Using DHCPv6Delegated Addressing in a Dual-Stack Network Using DHCPv6

DHCPv6 Prefix Delegation over PPPoE

The process of DHCPv6 prefix delegation when DHCPv6 is running over a PPPoE access network is as follows:

  1. The CPE obtains a link-local address by appending the interface ID that it receives through Internet Protocol version 6 Control Protocol (IPv6CP) negotiation to the IPv6 link-local prefix (FE80::/10). The link-local address provides an initial path for protocol communication between the BNG and CPE.

  2. The CPE sends a DHCPv6 solicit message that includes an IA_PD option.

  3. The BNG chooses a prefix for the CPE with information from an external authentication, authorization, and accounting (AAA) server or from a local prefix pool.

  4. The BNG sends an advertise message to the CPE. The message includes the delegated prefix, an IA_PD option, and an Identity Association for Prefix Delegation (IA_PD) prefix option. The prefix length in the IA_PD prefix option is 48. The message can also contain other configuration information, such as a maximum lease time.

  5. The CPE sends a request message to the BNG. The message requests the prefix that was advertised.

  6. The BNG returns the delegated prefix to the CPE in a reply message. This message also contains the delegated prefix, an IA_PD option, and an IA_PD prefix option. The prefix length in the IA_PD prefix option is 48. The message can also contain other configuration information, such as a maximum lease time.

  7. The CPE uses the delegated prefix to allocate global IPv6 addresses to host devices on the subscriber network. It can use router advertisements, DHCPv6, or a combination of these two methods to allocate addresses on the subscriber LAN.

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