150 Managing a Switch Stack
When the failed master resumes normal operation, it joins the stack as a
member (not a master) if the new stack master has already been elected.
The stack master copies its running configuration to the standby unit
whenever it changes (subject to some restrictions to reduce overhead). This
enables the standby unit to take over the stack operation with minimal
interruption if the stack master becomes unavailable.
Operational state synchronization also occurs:
• when you save the running configuration to the startup configuration on
the stack master.
• when the backup unit changes.
What is Nonstop Forwarding?
Networking devices 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.
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.
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