Concept Guide

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 Configuring Spanning Trees as Hitless.
Graceful Restart
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 EMC 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 EMC 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 EMC 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 EMC Networking OS fails over to standby stack unit. If any health checks on
a line card fail, Dell EMC Networking OS resets the card to bring it back to the correct state.
System Health Monitoring
Dell EMC 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.
Failure and Event Logging
Dell EMC Networking systems provide multiple options for logging failures and events.
Trace Log
Developers interlace messages with software code to track the execution of a program.
These messages are called trace messages and are primarily used for debugging and to provide lower-level information then
event messages, which system administrators primarily use. Dell EMC Networking OS retains executed trace messages for
hardware and software and stores them in files (logs) on the internal flash.
● NV Trace Log — contains line card bootup trace messages that Dell EMC Networking OS never overwrites and is stored in
internal flash under the directory NVTRACE_LOG_DIR.
● Trace Log — contains trace messages related to software and hardware events, state, and errors. Trace Logs are stored in
internal flash under the directory TRACE_LOG_DIR.
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High Availability (HA)