This section describes functionality of the prefix delegation from the RADIUS server.

During initial authentication process, RADIUS AAA can authorize Framed-IPv6-Address and Delegated-IPv6-Prefix AVP. Prior to the introduction of this feature Cisco P-GW was able to process only Framed-IPv6-Address AVP. This AVP was treated as Default-Prefix for the attaching UE. P-GW used to allocate 64-bit Interface-ID and the combined 128-bit address. The 64-bit default-prefix, derived from Framed-IPv6-Address AVP, and locally generated 64-bit Interface-ID was sent to the UE during this initial attachment. This 64-bit default-prefix was then associated with the default bearer/PDN for the UE and is considered as the UE's IPv6 address.

With the introduction of this feature, P-GW is able to process Delegated-IPv6-Prefix AVP along with Framed-IPv6-Address. Delegated-IPv6-Prefix AVP is used to designate the Delegated Prefix of prefix length 48/52/56 bits. This AVP is treated differently than Framed-IPv6-Address. P-GW communicates this delegated prefix to the UE only using DHCPv6 message handshake SOLICIT/ADVERTIZE/REQUEST/RESPONSE. Delegated-IPv6-Prefix is not associated with the default bearer and it is not considered as the UE IPv6 address.

1. Configure the APN on the P-GW, to enable the prefix delegation from the RADIUS server. For the configuration steps, see Configuring APN to Enable Prefix Delegation From RADIUS Server.

2. Configure APN on the P-GW for the prefix delegation. RADIUS server may send delegated prefix in the Access-Accept message independent of the APN configuration on the P-GW. Based on the APN configuration and presence of delegate prefix in the Access-Accept message, the following combinations are possible. The PDN setup is rejected if:

   • The RADIUS server has not sent Delegated Prefix in the Access-Accept message
   • The pd-alloc-method in the APN configuration is no-dynamic
   The following table lists all possible combination of the APN configuration and presence of delegated prefix in the Access-Accept message:

   Table 1. Mapping of APN Configuration and RADIUS Message

   pd-alloc-method in APN Configuration	Delegated-IPv6-Prefix in Access-Accept RADIUS Message	PDN State
   no-dynamic	Yes	PDN is set up if: The delegated prefix is successfully allocated after level1 and level2 validations are done Validation with the static pool, as mentioned in step 3, is successful If validation fails, PDN is not set up.
   no-dynamic	No	PDN is not set up.
   local/dhcpv6-proxy	Yes	Delegated-IPv6-Prefix in Access-Accept RADIUS message is discarded. PDN is set up. Delegate prefix is allocated to the UE, on receiving SOLICIT message, based on the configured pd-alloc-method in the APN.
   local/dhcpv6-proxy	No	PDN is set up. Delegated prefix is allocated to the UE, on receiving SOLICIT message, based on the configured pd-alloc-method in the APN.

3. The P-GW then performs the following two level validation for the prefix length received in Access-Accept RADIUS message:

   Level 1: Prefix length must be only one of the supported values, such as, 48 / 52 / 56. For any other length, delegate prefix is rejected and PDN is not set up.

   Level 2: If level 1 validation is passed, the prefix length is compared with the prefix-delegation-len configured in the APN using the CLI command, ipv6 address prefix-delegation-len.

   If there is a mismatch, delegate prefix is rejected and PDN is not set up.

   Important

   Level2 validation is not done if prefix-delegation-len is not configured in the APN.

4. Only if the above two level validation is successful, the received delegate prefix is validated against the static ipv6 prefix pool configured in the destination context. If validation with the static pool is successful, then the delegate prefix is stored on the P-GW. If validation with the static pool fails, the delegate prefix is rejected and PDN is not set up.

5. After the PDN is set up, the UE or the CPE sends a delegated prefix request by sending DHCPv6 SOLICIT message to the P-GW. P-GW sends the delegated prefix, which it had stored earlier, in the DHCPv6 ADVERTISE message to the UE.

6. Next, the UE sends the DHCPv6 REQUEST message to the P-GW and the P-GW sends the DHCPv6 REPLY message to the UE, which completes the DHCPv6 handshake.

   1. When the DHCPv6 RELEASE message is received from the UE, P-GW blocks data from any sources IP address from the delegated prefix pool. The delegated prefix is not released to the static ipv6 prefix pool from which it was allocated. If the DHCPv6 SOLICIT message is received again from the UE, the same delegate prefix is sent to the UE. The P-GW starts passing the data from the source address part of the said delegated prefix pool.

      DHCPv6 RELEASE REPLY message is sent to the UE, only when the UE requests delegated prefix release by sending DHCPv6 RELEASE REQUEST message to the P-GW.

      The DHCPv6 RELEASE REPLY message is not sent to the UE and no message is sent to the RADIUS server if:

      • The delegated prefix is released when validity time configured in the DHCPv6 service expires

      • When the PDN is cleared

      Release triggered reason can be checked from the DHCPv6 statistics(output of the CLI command show dhcpv6 statistics, which are as follows:

      
       Session Release Reasons:(dhcp-prefix-delegation) 
      PDNs Released: 3              Lease Exp Policy: 0 
       UE Initiated Release: 1      Other Reasons: 0 

7. When the PDN is cleared, the delegate prefix is released to the static ipv6 prefix pool from which it was allocated.

Use the following syntax to configure the APN profile on the GGSN/P-GW/SAEGW for enabling Prefix Delegation from the RADIUS Server.

config 
   context context_name 
      apn apn_name 
								 ipv6 address alloc-method [dhcpv6-proxy | local | no-dynamic ] allow-prefix-delegation pd-alloc-method no-dynamic 
									ipv6 address prefix-delegation-len [48 | 52 | 56]  
     				end 

Notes:

To verify the Prefix Delegation from the RADIUS Server, use the following show commands.

This section describes the functionality of the prefix delegation from the local pool.

1. Configure APN on the GGSN/P-GW/SAE-GW to enable the prefix delegation from the local pool. For configuration steps, see Configuring APN for Private Pool Name

2. Once the APN is configured, configure the pool on destination context. See Configuring Prefix Delegation on Destination Context.

3. The PDN is first set up with default ipv6 prefix of length 64.

4. Once the PDN is set up, User Equipment (UE) or a Customer Premises Equipment (CPE) can request delegated prefix by sending DHCPv6 SOLICIT message to P-GW.

5. The P-GW then performs the following two level validation for the prefix length:

   Level 1: The prefix length requested in DHCPv6 SOLICIT message must be only one of the supported values, 48 / 52 / 56. For any other length, the SOLICIT is silently dropped at P-GW.

   Level 2: If level 1 validation is successful, then the following validation is done. If prefix-delegation-len is configured in the APN, then delegate prefix allocation of this length is attempted from the local private pool. If prefix-delegation-len is not configured in the APN, then delegate prefix allocation of length requested in SOLICIT message is attempted from the local private pool.

   Important

   The requested length for the delegate prefix must match with the prefix-length configured for the private pool. The requested prefix length is as configured in the APN as ipv6 address prefix-delegation-len 52. If it is not configured in the APN, it may also be from the SOLICIT message. Configure the prefix length for the private pool by using the CLI command, ipv6 pool ipv6-private prefix 5001::1/48 prefix-length 52 private 0. Only when these lengths match, delegated prefix allocation from the local pool is successful.

6. The UE or a CPE can request the delegated prefix by sending DHCPv6 SOLICIT message to the P-GW. P-GW sends the delegated prefix allocated from the local pool, in the DHCPv6 ADVERTISE message to the UE.

7. Next, the UE sends the DHCPv6 REQUEST message to the P-GW. The P-GW sends the DHCPv6 REPLY message to the UE, which completed the DHCPv6 handshake.

   If the delegated prefix allocation from the local pool fails, the DHCPv6 SOLICIT message is silently dropped at the P-GW.

8. When the UE sends the DHCPv6 RELEASE message, the delegated prefix is released to the ipv6 prefix pool.

   DHCPv6 RELEASE REPLY message is sent to the UE, only when the UE requests prefix delegation released by sending DHCPv6 RELEASE REQUEST message to the P-GW.

   DHCPv6 RELEASE REPLY message is not sent to UE if:

   • The prefix delegation is released when validity time configured in the DHCPv6 service expires

   • The PDN is cleared

   If DHCPv6 SOLICIT message is received again from the UE, a new delegated prefix is allocated from the local pool and sent to the UE.

To verify the Prefix Delegation from the local pool, use the following show commands.

Following CLI command can be used to see the total current active counter for Interface-ID allocation.

IP address allocation statistics: 
Total IPv6 Interface IDs allocated: 
    AAA provided:        1 
    Locally Generated:   2