| Engineering Journal 5.5.1: ISDN commands 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 (that is, it is a TE2 device), it must use an external ISDN terminal adapter. On a TE2 router, use the command interface serial interface-number. Use the encapsulation ppp command if you want PPP encapsulation for your ISDN interface. This is the case if you want any of the rich LCP options that PPP offers (for example, CHAP authentication). You must use PPP PAP or CHAP if you will receive calls from more than one dialup source.