5.3 ISDN BRI and DDR
5.3.4 Step 2 - configuring the interface

The interface bri interface-number command designates the interface used for ISDN on a router acting as a TE1. If the router does not have a native BRI (it is a TE2 device), it must use an external ISDN terminal adapter. On a TE2 router, use the interface serial interface-number command.