management plane is application software running on the Management Unit
that provides interfaces allowing a network administrator to configure the
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 Management Unit. This type of operation is called nonstop
forwarding. When the Management Unit fails, only the switch ASICs on the
Management Unit 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
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 Management 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
Management Unit to reboot (cold start), and the new master to perform a
Initiating a failover reloads the Management unit, triggering the backup unit
to take over. Before the failover, the Management Unit 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
Managing a Switch Stack