Users Guide
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High Availability (HA)
High availability (HA) is a collection of features that preserves system continuity by maximizing uptime and minimizing packet loss
during system disruptions.
High Availability on Stacks
A stack has a master and standby management unit that provide redundancy in a similar way to redundant route processor modules
(RPMs).
The master unit synchronizes the running conguration and protocol states so that the system fails over in the event of a hardware
or software fault on the master unit.
In such an event, or when the master unit is removed, the standby unit becomes the stack manager and Dell Networking OS elects a
new standby unit. Dell Networking OS resets the failed master unit: after it is online, it becomes a member unit; the remaining
members remain online.
For more information about the failover process in an switch stack, refer to the Stacking chapter.
Hitless Behavior
Hitless is a protocol-based system behavior in a stacked S5000 switch that is transparent to remote systems. In the event of a stack
unit failover, it is not necessary to notify the remote systems of a local state change.
Hitless protocols are compatible with other hitless and graceful restart protocols. Hitless failovers may be triggered by a software or
hardware exception, or a forced failover via the CLI.
For example, if you congure hitless open shortest path rst (OSPF) 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 (refer to Link Aggregation Control Protocol (LACP)).
• Spanning tree protocol. (Refer to Conguring Spanning Trees as Hitless).
• Bi-directional Forwarding Detection (refer to Bidirectional Forwarding Detection (BFD))
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 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.
Dell Networking OS supports graceful restart for the following protocols:
• Border gateway protocol
• Open shortest path rst
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High Availability (HA)