Subscriber LAN Addressing with DHCPv6 Prefix Delegation

You can use DHCPv6 prefix delegation to automate the delegation of IPv6 prefixes to the CPE. With prefix delegation, a delegating router (the BNG) delegates IPv6 prefixes to a requesting router (the CPE). 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 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 DHCPv6

DHCPv6 prefix delegation operates as follows:

1. A delegating router is provided with IPv6 prefixes to be delegated to requesting routers. These prefixes can come from a local address-assignment pool or an external AAA server.

   Each prefix has an associated valid and preferred lifetime, which can be extended.

2. A requesting router requests one or more prefixes from the delegating router.

3. The delegating router chooses prefixes for delegation, and responds with prefixes to the requesting router.

4. The requesting router is then responsible for the delegated prefixes.

   The address allocation mechanism in the subscriber network can be performed with ICMPv6 Neighbor Discovery in router advertisements, DHCPv6, or a combination of these two methods.

For networks in which the service provider directly controls the CPE, a delegated prefix can be used to create an IPv6 address on the loopback interface between the CPE and the BNG. This address can be used to manage the CPE, and the CPE uses it as a source address when it communicates with the BNG.

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 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 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 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.

You can set up the BNG to select IPv6 prefixes to be delegated to the requesting router in one the following ways:

• An external source such as a AAA RADIUS server or a DHCP server using the DHCPv6 relay agent.

• Dynamic assignment from a local pool of prefixes that is configured on the BNG

  You can specify the name of a delegated pool to use for prefix delegation, which means that you do not need to use AAA to obtain the pool name. In this configuration, if you have also specified a pool match order, the specified delegated pool takes precedence.

Figure 2 shows how DHCPv6 prefix delegation is used in a dual-stack network.

Figure 2: Delegated Addressing in a Dual-Stack Network Using DHCPv6

DHCPv6 prefix delegation operates as follows:

1. A delegating router is provided with IPv6 prefixes to be delegated to requesting routers. These prefixes can come from a local address-assignment pool or an external AAA server.

   Each prefix has an associated valid and preferred lifetime, which can be extended.

2. A requesting router requests one or more prefixes from the delegating router.

3. The delegating router chooses prefixes for delegation, and responds with prefixes to the requesting router.

4. The requesting router is then responsible for the delegated prefixes.

   The address allocation mechanism in the subscriber network can be performed with ICMPv6 Neighbor Discovery Protocol (NDP) in router advertisements, DHCPv6, or a combination of these two methods.

The requesting router cannot use a sub-prefix of the delegated prefix assigned to it by the delegating router to the link between the delegating router and the requesting router. Because of this limitation, there are usually two routes to the CPE device. One is the delegated prefix, for the customer site behind the CPE device and the other for the link between the requesting router and the delegating router. To overcome this, Junos OS allows the exclusion of one specific prefix from a delegated prefix set while using DHCPv6 based prefix delegation as described in RFC 6603. This excluded prefix is used as the link between the delegating router and the requesting router. This prefix link is intended for use in networks where each requesting router is in its own Layer 2 domain.

To support prefix exclude delegation, the requesting router includes the Option Request option (ORO) with the PD_Exclude option in the solicit, request, renew, or rebind message to inform the delegating router about the support for the prefix delegation. When the Juniper Networks router acting as the DHCP server receives these message and finds the exclude prefix option (option 67) in ORO, it decides the prefix to be excluded. (The length of the prefix to be excluded is bigger than the delegated prefix length.) The excluded prefix is then added in the IA_Prefix options. The DHCP server acting as relay forwards the requested option to the server and relays the excluded prefix, assigned by the server, back to the client.

To exclude a prefix length in a DHCP server, configure the exclude-prefix-len statement at the [edit access address-assignment pool pool-name family dhcpv6 dhcp-attributes] hierarchy level. The length of the prefix can range from 1 through 128.

If the DHCP server supporting the exclude prefix wants the client to request for a prefix exclude after reconfiguration, then you can configure thesupport-option-pd-exclude statement either at the [edit system services dhcp-local-server dhcpv6] hierarchy level or at the [edit system services dhcp-local-server dhcpv6 group group-name] hierarchy level.