PPP callback provides a
client/server relationship between the endpoints of a point-to-point
connection. PPP callback allows a router to request that a dialup
peer router call back. The callback feature can be used to control
access and toll costs between the routers.
When PPP callback is configured on
the participating routers, the calling router (the callback client)
passes authentication information to the remote router (the callback
server), which uses the host name and dial string authentication
information to determine whether or not to place a return call. If the
authentication is successful, the callback server disconnects, and
then places a return call. The remote username of the return call is
used to associate it with the initial call, so that the packets can
be transmitted.
Both routers on a point-to-point link
must be configured for PPP callback; one must function as a callback
client, and one must be configured as a callback server. The
callback client must be configured to initiate PPP callback
requests, and the callback server must be configured to accept PPP
callback requests and place return calls.
Return calls are made through the same dialer
rotary group, but not necessarily the same line as the initial call.
A rotary group simplifies the configuration of multiple physical
interfaces by allowing you to apply a single logical interface
configuration to a set of physical interfaces. When many
destinations are configured, any of the physical interfaces in the
group can be used for outgoing calls. If the return call fails
(because the line is not answered or is busy), no retry
occurs. If the callback server has no interface available when
attempting the return call, it does not retry.
If the command dialer
hold-queue is configured on
the dialing-out interface, when the client router dials the initial
call, the router hold-queue timer is started . No calls to the same destination will be made again until
the hold-queue timer expires. The timer is stopped if PPP NCP
negotiation is successful or if the call fails. The complete command
syntax is as follows: