Black Box 16 port User Manual page 91

Challenge_ sets the interval in minutes at which the unit will issue a CHAP re-challenge to the
interval remote end. The default value is 0 (zero) meaning CHAP re-challenge is disabled.
During CHAP authentication an initial CHAP challenge takes place, and is unrelated
to CHAP re-challenges. The initial challenge takes place even if re-challenges are
disabled.
Some PPP client software does not work with CHAP re-challenges so you may wish
to leave the parameter disabled in the unit.
Black Box Console Server user guide
91