Cisco Systems 4500 OSPF Operation, IS-IS Operation

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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 9 Configuring Cisco NSF with SSO Supervisor Engine Redundancy About NSF with SSO Supervisor Engine Redundancy
OSPF Operation
When an OSPF NSF-capable router performs a supervisor engine switchover, it must perform the
following tasks in order to resynchronize its link state database with its OSPF neighbors:
• Relearn the available OSPF neighbors on the network without causing a reset of the neighbor
relationship
• Reacquire the contents of the link state database for the network
As quickly as possible after a supervisor engine switchover, the NSF-capable router sends an OSPF NSF
signal to neighboring NSF-aware devices. Neighbor networking devices recognize this signal as an
indicator 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 reestablished, 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. Once this exchange is complete, the NSF-capable device uses the routing
information to remove stale routes, update the RIB, and update the FIB with the new forwarding
information. The OSPF protocols are then fully converged.
Note OSPF support in NSF requires that all neighbor networking devices be NSF-aware. If an NSF-capable
router discovers that it has non-NSF -aware neighbors on a particular network segment, it disables NSF
capabilities for that segment. Other network segments composed entirely of NSF-capable or NSF-aware
routers continue to provide NSF capabilities.
IS-IS Operation
When an IS-IS NSF-capable router performs a supervisor engine switchover, it must perform the
following tasks in order to resynchronize its link state database with its IS-IS neighbors:
• Relearn the available IS-IS neighbors on the network without causing a reset of the neighbor
relationship
• Reacquire the contents of the link state database for the network
The IS-IS NSF feature offers two options when you configure NSF:
• Internet Engineering Task Force (IETF) IS-IS
• Cisco IS-IS
If neighbor routers on a network segment are running a software version that supports the IETF Internet
draft for router restartability, they assist an IETF NSF router that is restarting. With IETF, neighbor
routers provide adjacency and link-state information to help rebuild the routing information following a
switchover. A benefit of IETF IS-IS configuration is operation between peer devices based on a proposed
standard.
Note If you configure IETF on the networking device, but neighbor routers are not IETF-compatible, NSF
aborts following a switchover.
If the neighbor routers on a network segment are not NSF-aware, you must use the Cisco configuration
option. The Cisco IS-IS configuration transfers both protocol adjacency and link-state information from
the active to the redundant supervisor engine. An advantage of Cisco configuration is that it does not rely
on NSF-aware neighbors.