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Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter38 Configuring IP Unicast Routing
Configuring OSPF
OSPF NSF Awareness
The IP-services feature set supports OSPF NSF Awareness supported for IPv4. When the neighboring
router is NSF-capable, the Layer 3 switch continues to forward packets from the neighboring router
during the interval between the primary Route Processor (RP) in a router crashing and the backup RP
taking over, or while the primary RP is manually reloaded for a non-disruptive software upgrade.
This feature cannot be disabled. For more information on this feature, see the “OSPF Nonstop
Forwarding (NSF) Awareness” section of the Cisco IOS IP Routing Protocols Configuration Guide,
Release 12.4 at this URL:
http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a00804557
a8.html
OSPF NSF Capability
The Catalyst 3750-E IP-services feature set also supports OSPF NSF-capable routing for IPv4 for better
convergence and lower traffic loss following a stack master change. When a stack master change occurs
in an OSPF NSF-capable stack, the new stack master must do two things to resynchronize its link-state
database with its OSFP neighbors:
•Release the available OSPF neighbors on the network without resetting the neighbor relationship.
•Reacquire the contents of the link-state database for the network.
After a stack master change, the new master sends an OSPF NSF signal to neighboring NSF-aware
devices. A device recognizes this signal to mean that it should not reset the neighbor relationship with
the stack. As the NSF-capable stack master receives signals from other routes on t he network, it begins
to rebuild its neighbor list.
When the neighbor relationships are reestablished, the NSF-capable stack master resynchronizes its
database with its NSF-aware neighbors, and routing information is exchanged between the OSPF
neighbors. The new stack master uses this routing information to remove stale routes, to update the
routing information database (RIB), and to update the forwarding information base (FIB) with the new
information. The OSPF protocols then fully converge.
Note OSPF NSF requires that all neighbor networking devices be NSF-aware. If an NSF-capable router
discovers non-NSF aware neighbors on a network segment, it disables NSF capabilities for that segment.
Other network segments where all devices are NSF-aware or NSF-capable continue to provide NSF
capabilities.
Use the nsf OSPF routing configuration command to enable OSPF NSF routing. Use the show ip ospf
privileged EXEC command to verify that it is enabled.
For more information about this feature, see the Cisco Nonstop Forwarding Feature Overview at this
URL:
http://www.cisco.com/en/US/products/sw/iosswrel/ps1829/products_feature_guide09186a00800ab7fc.
html
Note NSF is not supported on interfaces configured for Hot Standby Router Protocol (HSRP).