3Com 3C13618 Command Reference Manual page 661

Security
that communication with the RADIUS server fails. Only when the radius method is
used will abnormality occur in authentication.
Five combination methods are listed below:
aaa authentication-scheme ppp default none
aaa authentication-scheme ppp default local
aaa authentication-scheme ppp default radius
aaa authentication-scheme ppp default radius none
aaa authentication-scheme ppp default radius local
Description
Using the aaa authentication-scheme ppp command, you can configure the AAA
PPP authentication method list. Using the undo aaa authentication-scheme ppp
command, you can cancel the AAA PPP authentication list.
By default, authentication method list configured.
CHAP or PAP in the process of PPP negotiation is only an authentication process, via
which such information as the username and password is interacted between the
customer end and the AAA server. Whether it can pass the authentication is
determined by whether the username or password provided by the customer matches
certain customer information record in the USER dictionary of the AAA authentication
server.
Three authentication methods can be specified in AAA's PPP authentication local,
radius and none. local is an authentication method via the local database, radius is
an authentication method via the RADIUS server, while none indicates that no
authentication is conducted.
The local database is configured through the local-user command.
As for non-default method lists, the undo aaa authentication-scheme ppp
command serves to cancel a method list; as for a default method list, this command
serves to restore the default method to a default state, that is, local authentication.
For related commands, see aaa authentication-scheme local-first, ppp
authentication-mode, local-user password, local-user callback-number,
local-user call-number, local-user ftp-directory, local-user service-type.
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