Fra Cisco:
http://www.cisco.com/warp/public/779/smbiz/service/configs/700/700_glossary.htmSEt PPp AUthentication OUtgoing [CHap] [PAp] [NOne] IOS-700 Command Reference
This command configures PPP authentication for outgoing ISDN calls. Enabling authentication on outgoing calls is known as bidirectional authentication. For bidirectional authentication, not only does the answering router authenticate the calling router, but the calling router also authenticates the answering router. In contrast, disabling authentication on outgoing calls (i.e. set ppp authentication outgoing none) is known as unidirectional authentication. In a unidirectional authentication scenario, only the answering router authenticates the calling router.
Cisco routers support both unidirectional and bidirectional authentication. For added security, bidirectional authentication is preferred and is used in all of the examples other than the ISP examples. In bidirectional examples, Atlanta places a call to Boston using the \"Boston\" profile. Upon connection, Atlanta supplies a PPP clientname (Atlanta) and a PPP secret client (gocisco1) configured in the Boston profile. Boston checks for a user profile that matches the received PPP clientname (Atlanta) and compares the received PPP client secret (gocisco1) against the PPP secret host (gocisco1) configured in the Atlanta profile. Likewise, Atlanta authenticates Boston in the same manner. With bidirectional authentication, Atlanta has the added security that the device answering the ISDN call is intended router.
Unidirectional authentication (i.e. set ppp authentication outgoing none) is used in the ISP sample configurations because non-Cisco routers that are potentially in use at an ISP do not support bidirectional authentication. When Atlanta calls the ISP, Atlanta does not force the ISP to identify itself with a username and password. However, the ISP authenticates Atlanta. Proponents of unidirectional authentication believe that, if Atlanta initiated the call, Atlanta must already know who it is calling.