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Implementing OSPF on Cisco IOS XR Software
Information About Implementing OSPF on Cisco IOS XR Software
RC-141
Cisco IOS XR Routing Configuration Guide
through intelligent line cards while the standby Route Processor (RP) assumes control from the failed
RP. The ability of line cards to remain up through a failover and to be kept current with the Forwarding
Information Base (FIB) on the active RP is key to Cisco IOS XR NSF operation.
Routing protocols, such as OSPF, run only on the active RP or DRP and receive routing updates from
their neighbor routers. When an OSPF NSF-capable router performs an RP failover, it must perform two
tasks to resynchronize its link-state database with its OSPF neighbors. First, it must relearn the available
OSPF neighbors on the network without causing a reset of the neighbor relationship. Second, it must
reacquire the contents of the link-state database for the network.
As quickly as possible after an RP failover, the NSF-capable router sends an OSPF NSF signal to
neighboring NSF-aware devices. This signal is in the form of a link-local LSA generated by the
failed-over router. Neighbor networking devices recognize this signal as a cue that the neighbor
relationship with this router should not be reset. As the NSF-capable router receives signals from other
routers on the network, it can begin to rebuild its neighbor list.
After neighbor relationships are re-established, the NSF-capable router begins to resynchronize its
database with all of its NSF-aware neighbors. At this point, the routing information is exchanged
between the OSPF neighbors. After this exchange is completed, the NSF-capable device uses the routing
information to remove stale routes, update the RIB, and update the FIB with the new forwarding
information. OSPF on the router as well as the OSPF neighbors are now fully converged.
Note The standardized IETF version of NSF, known as OSPF graceful restart (RFC 3623) is also supported.
Load Balancing in OSPF Version 2 and OSPFv3
When a router learns multiple routes to a specific network by using multiple routing processes (or
routing protocols), it installs the route with the lowest administrative distance in the routing table.
Sometimes the router must select a route from among many learned by using the same routing process
with the same administrative distance. In this case, the router chooses the path with the lowest cost (or
metric) to the destination. Each routing process calculates its cost differently; the costs may need to be
manipulated to achieve load balancing.
OSPF performs load balancing automatically. If OSPF finds that it can reach a destination through more
than one interface and each path has the same cost, it installs each path in the routing table. The only
restriction on the number of paths to the same destination is controlled by the maximum-paths (OSPF)
command. The default number of maximum paths is 32 for Cisco CRS-1 routers and 16 for
Cisco XR 12000 Series Routers. The range is from 1 to 32 for Cisco CRS-1 routers and 1 to 16 for
Cisco XR 12000 Series Routers.
Graceful Restart for OSPFv3
In the current release, various restart scenarios in the control plane of an IPv6-enabled router can disrupt
data forwarding. The OSPFv3 Graceful Restart feature can preserve the data plane capability in the
following circumstances:
• RP failure, resulting in a switchover to the backup processor
• Planned OSPFv3 process restart, such as software upgrade or downgrade
• Unplanned OSPFv3 process restart, such as a process crash
This feature supports non-stop data forwarding on established routes while the OSPFv3 routing protocol
is restarting. (Therefore, this feature enhances high availability of IPv6 forwarding.)