Service Manual

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.
Topics:
• High Availability on Stacks
• Hitless Behavior
• Graceful Restart
• Software Resiliency
• Hot-Lock Behavior
• Component Redundancy
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 configuration 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 configure hitless open shortest path first (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 flaps upon an RPM failover.
The following protocols are hitless:
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