Users Guide

248 Stacking
on the stack master. This type of operation is called nonstop forwarding
(NSF). When the stack master fails, only the switch ASICs and processor 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 across 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. In this case, the stack
master will attempt resynchronization no more often than once every 120
seconds.
The show nsf command output includes information about when the next
running-config synchronization will occur.
Initiating a Failover
The NSF feature allows the administrator to initiate a failover using the
initiate failover command. This method is preferred over the reload unit
command as it ensures synchronization of the stack master and standby unit.
Initiating a failover reloads the stack master, triggering the standby unit to
take over. Before the failover, the stack master pushes application data and
other important information to the standby unit. Although the handoff is
controlled and causes minimal network disruption, some ephemeral
application state is lost, such as pending timers and other pending internal
events. Use the show nsf command to view the stack checkpoint status prior
to reloading a stack member. Do not reload while a checkpoint operation is in
progress.
Always check the stack health before failing over to the standby unit. Use the
show switch stack-ports counters command to verify that the stack ports are
up and no errors are present. Resolve any error conditions prior to failing over