Users Guide

258 Stacking
Operational state synchronization also occurs:
• when the running configuration is saved to the startup configuration on
the stack master.
• when the standby unit changes.
What is Nonstop Forwarding?
Networking devices, such as the Dell EMC Networking N-Series switches, are
often described in terms of three semi-independent functions called the
forwarding plane, the control plane, and the management plane. The
forwarding plane forwards data packets and is implemented in hardware. The
control plane is the set of protocols that determine how the forwarding plane
should forward packets, deciding which data packets are allowed to be
forwarded and where they should go. 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
(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