We will dive into IPv6 prefix delegation prefix.

First of all, we will make a real simple topology :

R1 acts as a DHCP server and use the prefix delegation feature. But how it works ? How it is configured ?

R1 :

ipv6 unicast-routing
ipv6 cef
ipv6 dhcp pool POOLv6
 prefix-delegation pool p lifetime 180 120
 domain-name lucas.fr.eu.org

ipv6 local pool p 2001:DB8::/40 48

interface FastEthernet0/0
 ip address 10.0.0.1 255.255.255.0
 duplex half
 ipv6 address 2A02::1/48
 ipv6 enable
 ipv6 dhcp server POOLv6


R1#   show ipv6 dhcp interface 
FastEthernet0/0 is in server mode
  Using pool: POOLv6
  Preference value: 0
  Hint from client: ignored
  Rapid-Commit: disabled
R1#

R2 :

interface FastEthernet0/0
 duplex half
 ipv6 address autoconfig default
 ipv6 enable
 ipv6 dhcp client pd prefix-from-provider

interface FastEthernet1/1
 no ip address
 duplex auto
 speed auto
 ipv6 address prefix-from-provider ::1:0:0:0:1/64
 ipv6 enable

R2#show ipv6 dhcp interface 
FastEthernet0/0 is in client mode
  Prefix State is OPEN
  Renew will be sent in 00:00:04
  Address State is IDLE
  List of known servers:
    Reachable via address: FE80::C805:ADFF:FE80:0
    DUID: 00030001CA05AD800000
    Preference: 0
    Configuration parameters:
      IA PD: IA ID 0x00040001, T1 60, T2 120
        Prefix: 2001:DB8::/48
                preferred lifetime 120, valid lifetime 180
                expires at May 03 2016 10:53 PM (125 seconds)
      Domain name: lucas.fr.eu.org
      Information refresh time: 0
  Prefix name: prefix-from-provider
  Prefix Rapid-Commit: disabled
  Address Rapid-Commit: disabled
R2#

Debug trace on R2 (debug ipv6 dhcp) :

*May  3 22:36:11.859: IPv6 DHCP: Sending RENEW to FF02::1:2 on FastEthernet0/0
*May  3 22:36:11.859: IPv6 DHCP: DHCPv6 changes state from OPEN to RENEW (TIMEOUT) on FastEthernet0/0
*May  3 22:36:11.879: IPv6 DHCP: Received REPLY from FE80::C805:ADFF:FE80:0 on FastEthernet0/0
*May  3 22:36:11.879: IPv6 DHCP: Processing options
*May  3 22:36:11.879: IPv6 DHCP: Adding prefix 2001:DB8::/48 to prefix-from-provider
*May  3 22:36:11.883: IPv6 DHCP: T1 set to expire in 60 seconds
*May  3 22:36:11.883: IPv6 DHCP: T2 set to expire in 120 seconds
*May  3 22:36:11.883: IPv6 DHCP: DHCPv6 changes state from RENEW to OPEN (REPLY_RECEIVED) on FastEthernet0/0

We have acquired the prefix via PD aka Prefix Delegation feature :

R2#show ipv6 general-prefix 
IPv6 Prefix prefix-from-provider, acquired via DHCP PD
  2001:DB8::/48 Valid lifetime 158, preferred lifetime 98
   FastEthernet1/1 (Address command)
R2#

On R3 or R4 :

interface FastEthernet0/0
 no ip address
 duplex half
 ipv6 address autoconfig default
 ipv6 enable
end


2#show ipv6 dhcp interface 
FastEthernet0/0 is in client mode
  Prefix State is OPEN
  Renew will be sent in 00:00:04
  Address State is IDLE
  List of known servers:
    Reachable via address: FE80::C805:ADFF:FE80:0
    DUID: 00030001CA05AD800000
    Preference: 0
    Configuration parameters:
      IA PD: IA ID 0x00040001, T1 60, T2 120
        Prefix: 2001:DB8::/48
                preferred lifetime 120, valid lifetime 180
                expires at May 03 2016 10:53 PM (125 seconds)
      Domain name: lucas.fr.eu.org
      Information refresh time: 0
  Prefix name: prefix-from-provider
  Prefix Rapid-Commit: disabled
  Address Rapid-Commit: disabled
R2#

If we debug we will see (debug ipv6 interface, debug ipv6 dhcp, debug ipv6 nd) :

May  3 22:05:01.335: ICMPv6-ND: Neighbour FE80::C806:ADFF:FE81:1D on FastEthernet0/0 : LLA ca06.ad81.001d
*May  3 22:05:01.335: ICMPv6-ND: INCMP -> STALE: FE80::C806:ADFF:FE81:1D
*May  3 22:05:01.335: IPv6-Address: intfid_algo is notactive on intf 4
*May  3 22:05:01.339: IPv6-Address: intfid_algo is active on intf 4
*May  3 22:05:01.339: IPv6-Address: Generating IntfID rc 0, prefix: 2001:DB8:0:1::/64, address 2001:DB8:0:1:C808:ADFF:FE85:0
*May  3 22:05:01.343: IPv6-Address: Prefix Information change for 2001:DB8:0:1::/64, 0x0 -> 0x1E0
*May  3 22:05:01.343: IPv6-Address: Adding prefix 2001:DB8:0:1::/64 to FastEthernet0/0
*May  3 22:05:01.343: IPv6-Address: Adding operating owner prefix configured on FastEthernet0/0
*May  3 22:05:01.347: IPv6-Address: Adding operating owner address configured on FastEthernet0/0
*May  3 22:05:01.347: IPv6-Address: Address 2001:DB8:0:1:C808:ADFF:FE85:0 configured on FastEthernet0/0
*May  3 22:05:01.347: IPv6-Addrmgr-
R4(config-if)#ND: DAD request for 2001:DB8:0:1:C808:ADFF:FE85:0 on FastEthernet0/0
*May  3 22:05:01.347: ICMPv6-ND: Sending NS for 2001:DB8:0:1:C808:ADFF:FE85:0 on FastEthernet0/0
*May  3 22:05:01.351: ICMPv6-ND: Autoconfiguring 2001:DB8:0:1:C808:ADFF:FE85:0 on FastEthernet0/0
*May  3 22:05:02.351: IPv6-Addrmgr-ND: DAD: 2001:DB8:0:1:C808:ADFF:FE85:0 is unique.
*May  3 22:05:02.351: ICMPv6-ND: Sending NA for 2001:DB8:0:1:C808:ADFF:FE85:0 on FastEthernet0/0
*May  3 22:05:02.355: IPv6-Address: Address 2001:DB8:0:1:C808:ADFF:FE85:0/64 is up on FastEthernet0/0

Finally, we are able to ping the DHCPv6 server :

R4#ping ipv6 2A02::1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2A02::1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/45/96 ms
R4#traceroute 2A02::1

Type escape sequence to abort.
Tracing the route to 2A02::1

  1 2001:DB8:0:1::1 12 msec 36 msec 12 msec
  2 2A02::1 8 msec 56 msec 36 msec
R4#