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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 E-Series: Failovers triggered by software exception, hardware exception, forced failover via the
CLI, and manual removal of the primary RPM are all hitless.
• On the C-Series: 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
OSPF is configured over hitless LACP LAGs, both features work seamlessly to deliver a hitless
OSPF-LACP result. However, if hitless behavior involves multiple protocols, all must be hitless in order to
achieve a hitless end result. For example, if OSPF is hitless but BFD is not, OSPF operates hitlessly and
BFD flaps upon an RPM failover.
The following protocols are hitless:
• Link Aggregation Control Protocol. See Configure LACP as Hitless on page 549.
• Spanning Tree Protocol. See Configuring Spanning Trees as Hitless on page 1064.
• On the E-Series only, Bi-directional Forwarding Detection (line card ports). See Bidirectional
Forwarding Detection on page 169.
Graceful Restart
Graceful Restart is supported on platform: e c s
Graceful restart (also called 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 E-Series, when you configure graceful restart, the system drops
no packets during an RPM failover for protocol-relevant destinations in the forwarding table, and is
therefore called “hitless”. On the C-Series and S-Series, packet loss is non-zero, but trivial, and so is still
called hitless.
FTOS supports graceful restart for the following protocols:
• Border Gateway Protocol. See Enable graceful restart on page 241.
• Open Shortest Path First. Graceful Restart on page 700.
• Protocol Independent Multicast—Sparse Mode. PIM-SM Graceful Restart on page 764.
• Intermediate System to Intermediate System. Chapter 23, “Intermediate System to Intermediate
System,” on page 507.
Software Resiliency
During normal operations FTOS monitors the health of both hardware and software components in the
background to identify potential failures, even before these failures manifest.
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