Dell PowerConnect 8024 User Configuration Manual page 148

Application software on the stack master acts as the control plane. The
management plane is application software running on the stack master that
provides interfaces allowing a network administrator to configure the device.
The Nonstop Forwarding (NSF) feature allows the forwarding plane of stack
units to continue to forward packets while the control and management
planes restart as a result of a power failure, hardware failure, or software fault
on the stack master. This type of operation is called nonstop forwarding.
When the stack master fails, only the switch ASICs on the stack master need
to be restarted.
To prevent adjacent networking devices from rerouting traffic around the
restarting device, the NSF feature uses the following three techniques:
1 A protocol can distribute a part of its control plane to stack units so that
the protocol can give the appearance that it is still functional during the
restart.
2 A protocol may enlist the cooperation of its neighbors through a technique
known as graceful restart.
3 A protocol may simply restart after the failover if neighbors react slowly
enough that they will not normally detect the outage.
The NSF feature enables the stack master unit to synchronize the running-
config within 60 seconds after a configuration change has been made.
However, if a lot of configuration changes happen concurrently, NSF uses a
back-off mechanism to reduce the load on the switch. The show nsf
command output includes information about when the next running-config
synchronization will occur.
Initiating a Failover
The NSF feature allows you to initiate a failover, which causes the former
stack master to reboot (cold start), and the new master to perform a warm
restart.
Initiating a failover reloads the stack master, triggering the backup unit to
take over. Before the failover, the stack master pushes application data and
other important information to the backup unit. Although the handoff is
controlled and causes minimal network disruption, some application state is
lost, such as pending timers and other pending internal events.
148
Managing a Switch Stack