Configuring AAA preauthentication with ISDN PRI or channel-associated signaling (CAS) allows service providers to better
manage ports using their existing RADIUS solutions and efficiently manage the use of shared resources to offer differing service-level
agreements. With ISDN PRI or CAS, information about an incoming call is available to the network access server (NAS) before
the call is connected. The available call information includes the following:
The DNIS number, also referred to as the called number
The Calling Line Identification (CLID) number, also referred to as the calling number
The call type, also referred to as the bearer capability
The AAA preauthentication feature allows a Cisco NAS to decide--on the basis of the DNIS number, the CLID number, or the
call type--whether to connect an incoming call. (With ISDN PRI, it enables user authentication and authorization before a
call is answered. With CAS, the call must be answered; however, the call can be dropped if preauthentication fails.)
When an incoming call arrives from the public network switch, but before it is connected, AAA preauthentication enables the
NAS to send the DNIS number, CLID number, and call type to a RADIUS server for authorization. If the server authorizes the
call, the NAS accepts the call. If the server does not authorize the call, the NAS sends a disconnect message to the public
network switch to reject the call.
In the event that the RADIUS server application becomes unavailable or is slow to respond, a guard timer can be set in the
NAS. When the timer expires, the NAS uses a configurable parameter to accept or reject the incoming call that has no authorization.
The AAA preauthentication feature supports the use of attribute 44 by the RADIUS server application and the use of RADIUS
attributes that are configured in the RADIUS preauthentication profiles to specify preauthentication behavior. They can also
be used, for instance, to specify whether subsequent authentication should occur and, if so, what authentication method should
The following restrictions apply to AAA preauthentication with ISDN PRI and CAS:
Attribute 44 is available for CAS calls only when preauthentication or resource pooling is enabled.
Multichassis Multilink PPP (MMP) is not available with ISDN PRI.
AAA preauthentication is available only on some hardware platforms.
ISDN PRI is supported only on some hardware platforms.