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

Chapter 7 Configuring OSPFv3
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 .
Typically, you would summarize at the boundaries of area border routers (ABRs). Although you could
configure summarization between any two areas, it is better to summarize in the direction of the
backbone so that the backbone receives all the aggregate addresses and injects them, already
summarized, into other areas. The two types of summarization are as follows:
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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 OSPFv3 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. OSPFv3 supports stateful restart,
which is also referred to as non-stop routing (NSR). If OSPFv3 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, OSPFv3 attempts a graceful restart.
A graceful restart, or non-stop forwarding (NSF), allows OSPFv3 to remain in the data forwarding path
through a process restart. When OSPFv3 needs to perform a graceful restart, it sends a link-local Grace
(type 11) LSA. This restarting OSPFv3 platform is called NSF capable.
The Grace LSA includes a grace period, which is a specified time that the neighbor OSPFv3 interfaces
hold onto the LSAs from the restarting OSPFv3 interface. (Typically, OSPFv3 tears down the adjacency
and discards all LSAs from a down or restarting OSPFv3 interface.) The participating neighbors, which
are called NSF helpers, keep all LSAs that originate from the restarting OSPFv3 interface as if the
interface was still adjacent.
When the restarting OSPFv3 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
Inter-area route summarization
External route summarization
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 ospfv3 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
Information About OSPFv3
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