Delegated prefix length

The delegated prefix length (DPL) is applicable to subscriber-hosts with IPv6 Prefix (IA-PD) assigned by the DHCPv6 Server. An IPv6 prefix is more similar to a route than it is to an IP address. The length of the prefix plays crucial role in forwarding decisions, antispoofing, and prefix assignment through DHCPv6 pools in the local DHCPv6 server.

The structure of an IPv6 prefix is shown in Figure: IPv6 prefix.

For example, a DHCPv6 server prefix pool contains an aggregated (configured) IPv6 prefix from which the delegated prefixes are carved out. In Figure: IPv6 prefix this aggregated IPv6 prefix has length of /48. In addition, the DHCPv6 server needs to know the length of the delegated prefix (in the above case /60). These two values are marking the boundary within which a unique delegated prefix is selected.

The delegated prefix length can be obtained using:

• RADIUS

  • Delegated-IPv6-Prefix attribute that contains the prefix and the length (Delegated-IPv6-Prefix = AAAA:BBBB::/56). The DPL in this case is /56.

  • Alc-Delegated-IPv6-Prefix-Length VSA (to be used in conjunction with the DHCPv6 pool name - Alc-Delegated-IPv6-Pool VSA)

• LUDB

  Configured by LUDB per IPoEv6/PPPoEv6 host:

  This is to be used along with the DHCPv6 pool name (ipv6-delegated-prefix-pool) defined under the same CLI hierarchy.

  CLI syntax:

          configure
          subscriber-mgmt 
              local-user-db <name>
                  ipoe | ppp
                      host <name> 
                          ipv6-delegated-prefix-length [48 to 64]
  

  Alternatively, the entire prefix, including the DPL can be returned by LUDB.

  CLI syntax:

      configure
          subscriber-mgmt 
              local-user-db <name>
                      ipoe | ppp
                          host <name> 
                              ipv6-delegated-prefix <ipv6-prefix/prefix-length>
  

• DHCPv6 server

  Each DHCPv6 pool can optionally be configured with a DPL

  CLI syntax:

      configure
           service/router   
              dhcp6
                  local-dhcp-server <name>
                      pool <pool-name>
                          delegated-prefix-length [48 to 127]
  

Configured statically under the ipv6 CLI node of subscriber interface. In this case, the DPL is fixed for all subscriber hosts under the subscriber interface.

CLI syntax:

    configure
        service ies/vprn 
            subscriber-interface <ip-int-name>
                    ipv6
                        delegated-prefix-length [48 to 64] | variable