The value tmnxPppoePlcyPppAuthentication specifies what PPP
authentication protocol(s) will be negotiated during the LCP phase.
This field is only applicable if PAP/CHAP authentication is enabled
via configuration of a RADIUS authentication policy or a local user
database.
If set to 'pap' or 'chap', only PAP respectively CHAP is negotiated.
If set to 'prefChap', first CHAP is negotiated followed by PAP.
If set to 'prefPap', first PAP is negotiated followed by CHAP.