Reference Guide

Hitless Behavior
Hitless behavior is supported only on the S4820T platform.
Hitless is a protocol-based system behavior that makes an RPM failover on the local system transparent
to remote systems. The system synchronizes protocol information on the standby and primary RPMs
such that, in the event of an RPM failover, it is not necessary to notify the remote systems of a local state
change.
Hitless behavior is defined in the context of an RPM failover only and does not include line card, SFM, and
power module failures.
• On the S4820T platform: 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 first (OSPF) is configured 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.
• Spanning tree protocol. Refer to Configuring Spanning Trees as Hitless.
Graceful Restart
Graceful restart is supported on the S4820T 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 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. On the S4820T platform, 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 first
• Protocol independent multicast — sparse mode
• Intermediate system to intermediate system
High Availability (HA)
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