Managing a Switch Stack 153What is Stacking Standby?A standby unit is preconfigured in the stack. If the current stack master fails,the standby unit becomes the stack master. If no switch is pre-configured asthe standby unit, the software automatically selects a standby unit from theexisting stack units.When the failed master resumes normal operation, it joins the stack as amember (not a master) if the new stack master has already been elected.The stack master copies its running configuration to the standby unitwhenever it changes (subject to some restrictions to reduce overhead). Thisenables the standby unit to take over the stack operation with minimalinterruption if the stack master becomes unavailable.Operational state synchronization also occurs:• when you save the running configuration to the startup configuration onthe stack master.• when the backup unit changes.What is Nonstop Forwarding?Networking devices are often described in terms of three semi-independentfunctions called the forwarding plane, the control plane, and themanagement plane. The forwarding plane forwards data packets and isimplemented in hardware. The control plane is the set of protocols thatdetermine how the forwarding plane should forward packets, deciding whichdata packets are allowed to be forwarded and where they should go.Application software on the stack master acts as the control plane. Themanagement plane is application software running on the stack master thatprovides interfaces allowing a network administrator to configure the device.The Nonstop Forwarding (NSF) feature allows the forwarding plane of stackunits to continue to forward packets while the control and managementplanes restart as a result of a power failure, hardware failure, or software faulton the stack master. This type of operation is called nonstop forwarding.When the stack master fails, only the switch ASICs on the stack master needto be restarted.To prevent adjacent networking devices from rerouting traffic around therestarting device, the NSF feature uses the following three techniques:
