Graceful Restart; Software Resiliency; Software Component Health Monitoring; System Health Monitoring - Dell S4820T Configuration Manual

Standby stack units such that, in the event of a stack unit failover, it is not necessary 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) is configured over hitless the link aggregation control protocol (LACP) link
aggregation groups (LAGs), both features work seamlessly to deliver a hitless OSPF-LACP result.
However, to achieve a hitless end result, if the hitless behavior involves multiple protocols, all protocols
must be hitless. For example, if OSPF is hitless but bidirectional forwarding detection (BFD) is not, OSPF
operates hitlessly and BFD flaps upon an RPM failover.
The following protocols are hitless:
• Link aggregation control protocol.
• Spanning tree protocol. Refer to

Graceful Restart

Graceful restart is supported on the S4820T platform.
Graceful restart (also known as non-stop forwarding) is a protocol-based mechanism that preserves the
forwarding table of the restarting router and its neighbors for a specified period to minimize the loss of
packets. A graceful-restart router does not immediately assume that a neighbor 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 system

Software Resiliency

During normal operations, Dell Networking OS monitors the health of both hardware and software
components in the background to identify potential failures, even before these failures manifest.

Software Component Health Monitoring

On each of the line cards and the stack unit, there are a number of software components. Dell
Networking OS performs a periodic health check 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 Monitoring

Dell 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, and after exceeding a threshold can be used to initiate recovery mechanism.
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Configuring Spanning Trees as Hitless.
High Availability (HA)