Reference Guide

Table Of Contents
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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 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:
Link aggregation control protocol (refer to Link Aggregation Control Protocol (LACP)).
Spanning tree protocol. (Refer to Configuring Spanning Trees as Hitless).
Bi-directional Forwarding Detection (refer to Bidirectional Forwarding Detection (BFD))
High Availability (HA)
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