When the PPP Multilink is configured on an ISDN link and the dialer threshold is set to 0,
in order to raise both the B channels at the same time, the link is brought up but never brought down again,
like the dialer idle timeout is set to an infinite value.
When the threshold is put to 1 (1% load), the router behaves normally.
The solution for this problem is to configure the dialer threshold to 1 (the minimum value greater than 0)
1) If you want to use "Idle", it is needless to configure as "threshold 0", It should be configure threshold to 1 or others.
2) The channel B will not bring down when configure threshold as 0, It is the mechanism designed for our router and can not be changed.