Dell Networking N4000 Series Configuration Manual page 183

two fixed stacking ports in the rear of the switch. Stacking on Ethernet ports
is not supported. The fixed stacking ports show as TwentygigabitStacking and
are abbreviated Tw.
NSF is enabled by default. You can disable NSF to redirect the CPU resources
consumed by data checkpointing. Checkpointing only occurs when a backup
unit is elected, so there is no need to disable the NSF feature on a standalone
switch. When a new unit is added to the stack, the new unit takes the
configuration of the stack, including the NSF setting. OSPF implements a
separate graceful restart control that enables NSF for OSPF. OSPF graceful
restart is not enabled by default.
183
Managing a Switch Stack