Netgear XSM4324FS CLI Manual Software Version 12.x - Page 49

M4300 Intelligent Edge Series Fully Managed Stackable Switches Application software on the management unit functions as the control plane. The management plane is also application software that runs on the management unit and that provides interfaces, allowing you to configure and monitor the device. Nonstop forwarding (NSF) 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 management unit. You can also manually initiate a nonstop forwarding failover by issuing the initiate failover command. If the management unit fails, traffic flows that enter and exit the stack through physical ports on a unit other than the management unit continue with at most a subsecond interruption. To prepare the backup management unit for a failover, applications on the management unit continuously checkpoint (that is, forward) information to the backup unit. Changes to the running configuration are automatically copied to the backup unit. MAC addresses stay the same across a nonstop forwarding failover so that neighbors do not need to relearn them. When a nonstop forwarding failover occurs, the control plane on the backup unit starts from a partially-initialized state and applies the checkpointed (that is, forwarded) information. While the control plane is initializing, the stack cannot react to external changes, such as network topology changes. When the control plane is fully operational on the new management unit, the control plane ensures that the hardware state is updated as necessary. The control plane failover time depends on the size of the stack, the complexity of the configuration, and the speed of the CPU. The management plane restarts when a failover occurs. Management connections must be reestablished. For NSF to be effective, adjacent networking devices must not reroute traffic around the restarting device. The switch uses three protocol techniques to prevent traffic from being rerouted: • 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. Spanning tree and port channels use this technique. • A protocol can enlist the cooperation of its neighbors through a technique known as graceful restart. OSPF uses graceful restart if it is enabled (see "IP Event Dampening Commands on page 745). • A protocol can simply restart after the failover if neighbors react slowly enough that they do not detect the outage. The IP multicast routing protocols are a good example of this behavior. To take full advantage of nonstop forwarding, layer 2 connections to neighbors must be configured over port channels that span two or more stack units and layer 3 routes must be configured over ECMP routes with next hops over physical ports on two or more units. The hardware can quickly move traffic flows from port channel members or ECMP paths on a failed unit to a surviving unit. Stacking Commands 49 CLI Command Reference Manual