A subscriber in the service provider’s network receives an IPv6 prefix from the RADIUS server in the initial authentication
access-accept response. The broadband remote access server (BRAS) performs a uniqueness check to ensure that the IPv6 prefix
has not been assigned to another subscriber. The BRAS then receives an Internet Protocol Control Protocol (IPCP) request for
an IPv4 address from the subscriber. Next, the BRAS adds a number of attributes including the subscriber username and the
Cisco vendor-specific attribute (VSA) for IPv4 address saving to the request, and sends this information to the RADIUS server.
The VSA information tells the RADIUS server that this is a request for an existing session and the username identifies the
subscriber making the request. The RADIUS server then sends an IPv4 address in the access-accept response. The BRAS checks
that the IPv4 address that is to be assigned is not being used by any other subscriber. If a duplicate address is found, the
session is torn down, otherwise the session is authorized. If the subscriber sends another request for an IPv4 address without
terminating the earlier session, the BRAS does not send this request to the RADIUS server; instead, it uses the IPv4 address
returned in the previous authorization exchange. When the subscriber terminates the session, the BRAS releases the IPv4 address
and resets the authorization flag for this subscriber. This ensures that if the same subscriber requests an IPv4 address again,
the request will be forwarded to the RADIUS server.
In addition, we recommend that you configure the following features on the BRAS. For a detailed description of the commands
required to configure these features, see the
Cisco IOS Broadband Access Aggregation and DSL Command Reference
None of these features are mandatory for the IPv4 address conservation feature to work.