Reference Guide

414 | High Availability
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Graceful Restart
Graceful Restart is supported on platforms: 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.
• Open Shortest Path First.
• Protocol Independent Multicast—Sparse Mode.
• Intermediate System to Intermediate System.
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.
Runtime System Health Check
Runtime System Health Check is supported on platform: e
FTOS runs a system health check to detect data transfer errors within the system. FTOS performs the check
during normal operation by interspersing test frames among the data frames that carry user and system
data. One such check is a data plane loopback test.
There are some differences between the TeraScale and ExaScale line card and RPM testing:
• The TeraScale card test contains a loopback from the RPM to the SFM and a loopback from the line
cards to the SFM.
• The ExaScale card test contains a loopback from the RPM to the SFM and a loopback from the line
cards to the on-board TSF3.
• For TeraScale, each line card and RPM periodically sends out test frames that loop back through the
SFM. The loopback health check determines the overall status of the backplane and can identifies a
faulty SFM. If three consecutive RPM loopbacks fail, then the software initiates a fault isolation
procedure that sequentially disables one SFM at a time and performs the same loopback test.
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