Hitless BehaviorHitless is a protocol-based system behavior that makes a stack unit failover on the local system transparent to remote systems. The systemsynchronizes protocol information on the Management and Standby stack units such that, in the event of a stack unit failover, it is notnecessary to notify the remote systems of a local state change.Hitless behavior is defined in the context of a stack unit failover only.• Only failovers via the CLI are hitless. The system is not hitless in any other scenario.Hitless protocols are compatible with other hitless and graceful restart protocols. For example, if hitless open shortest path first (OSPF) isconfigured over hitless the link aggregation control protocol (LACP) link aggregation groups (LAGs), both features work seamlessly todeliver a hitless OSPF-LACP result. However, to achieve a hitless end result, if the hitless behavior involves multiple protocols, all protocolsmust be hitless. For example, if OSPF is hitless but bidirectional forwarding detection (BFD) is not, OSPF operates hitlessly and BFD flapsupon an RPM failover.The following protocols are hitless:• Link aggregation control protocol.• Spanning tree protocol. Refer to Configuring Spanning Trees as Hitless.Graceful RestartGraceful restart (also known as non-stop forwarding) is a protocol-based mechanism that preserves the forwarding table of the restartingrouter and its neighbors for a specified period to minimize the loss of packets. A graceful-restart router does not immediately assume that aneighbor is permanently down and so does not trigger a topology change. Packet loss is non-zero, but trivial, and so is still called hitless.Dell Networking OS supports graceful restart for the following protocols:• Border gateway• Open shortest path first• Protocol independent multicast — sparse mode• Intermediate system to intermediate systemSoftware ResiliencyDuring normal operations, Dell Networking OS monitors the health of both hardware and software components in the background toidentify potential failures, even before these failures manifest.Software Component Health MonitoringOn each of the line cards and the stack unit, there are a number of software components. Dell Networking OS performs a periodic healthcheck on each of these components by querying the status of a flag, which the corresponding component resets within a specified time.If any health checks on the stack unit fail, the Dell Networking OS fails over to standby stack unit. If any health checks on a line card fail,Dell Networking OS resets the card to bring it back to the correct state.System Health MonitoringDell Networking OS also monitors the overall health of the system.Key parameters such as CPU utilization, free memory, and error counters (for example, CRC failures and packet loss) are measured, andafter exceeding a threshold can be used to initiate recovery mechanism.High Availability (HA) 367
