High Availability And Graceful Restart - Cisco Nexus 7000 Series Configuration Manual

Chapter 6 Configuring OSPFv2
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .
You configure inter-area route summarization on ABRs, summarizing routes between areas in the
autonomous system. To take advantage of summarization, you should assign network numbers in areas
in a contiguous way to be able to lump these addresses into one range.
External route summarization is specific to external routes that are injected into OSPFv2 using route
redistribution. You should make sure that external ranges that are being summarized are contiguous.
Summarizing overlapping ranges from two different routers could cause packets to be sent to the wrong
destination. Configure external route summarization on ASBRs that are redistributing routes into OSPF.
When you configure a summary address, Cisco NX-OS automatically configures a discard route for the
summary address to prevent routing black holes and route loops.

High Availability and Graceful Restart

Cisco NX-OS provides a multilevel high-availability architecture. OSPFv2 supports stateful restart,
which is also referred to as non-stop routing (NSR). If OSPFv2 experiences problems, it attempts to
restart from its previous run-time state. The neighbors do not register any neighbor event in this case. If
the first restart is not successful and another problem occurs, OSPFv2 attempts a graceful restart.
A graceful restart, or nonstop forwarding (NSF), allows OSPFv2 to remain in the data forwarding path
through a process restart. When OSPFv2 needs to perform a graceful restart, it sends a link-local opaque
(type 9) LSA, called a grace LSA (see the
platform is called NSF capable.
The grace LSA includes a grace period, which is a specified time that the neighbor OSPFv2 interfaces
hold onto the LSAs from the restarting OSPFv2 interface. (Typically, OSPFv2 tears down the adjacency
and discards all LSAs from a down or restarting OSPFv2 interface.) The participating neighbors, which
are called NSF helpers, keep all LSAs that originate from the restarting OSPFv2 interface as if the
interface was still adjacent.
When the restarting OSPFv2 interface is operational again, it rediscovers its neighbors, establishes
adjacency, and starts sending its LSA updates again. At this point, the NSF helpers recognize that the
graceful restart has finished.
Stateful restart is used in the following scenarios:
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Graceful restart is used in the following scenarios:
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OL-20002-02
First recovery attempt after the process experiences problems
ISSU
User-initiated switchover using the system switchover command
Second recovery attempt after the process experiences problems within a 4-minute interval
Manual restart of the process using the restart ospf command
Active supervisor removal
Active supervisor reload using the reload module active-sup command
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
"Opaque LSAs" section on page
Information About OSPFv2
6-7). This restarting OSPFv2
6-11