Service Manual
Hitless behavior is dened 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 rst
(OSPF) is congured 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 aps upon an RPM failover.
The following protocols are hitless:
• Link aggregation control protocol.
• Spanning tree protocol. Refer to Conguring Spanning Trees as Hitless.
Graceful Restart
Graceful restart is supported on the 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 specied 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 rst
• 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 ag, which the corresponding component resets within a
specied 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.
High Availability (HA)
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