You definately need a dialer-group and a dialer list
to set the intresting traffic. If not used what will happen is
the interface dialer would bring the isdn link up
when the primary interface goes down but after the idle timeout
of 120sec because no traffic as been set as intresting. The idle
timeout would not be reset and it would bring down the isdn connection.
The theory of classifying packets as intresting
is not only for dial initiation but also to
reset the idle-timeout and keep the link up as
long as the set intresting traffic is still flowing over the link.