Table Of Contents

OSPF Incremental SPF

Contents

Prerequisites for OSPF Incremental SPF

Information About OSPF Incremental SPF

Benefits of OSPF Incremental SPF

How to Enable OSPF Incremental SPF

Enabling Incremental SPF

Configuration Examples for OSPF Incremental SPF

Incremental SPF: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

OSPF Incremental SPF

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The Open Shortest Path First (OSPF) protocol can be configured to use an incremental SPF algorithm for calculating the shortest path first routes. Incremental SPF is more efficient than the full SPF algorithm, thereby allowing OSPF to converge faster on a new routing topology in reaction to a network event.

Feature History for the OSPF Incremental SPF Feature

Release	Modification
12.0(24)S	This feature was introduced.
12.3(2)T	This feature was integrated into Cisco IOS Release 12.3(2)T.
12.2(18)S	This feature was integrated into Cisco IOS Release 12.2(18)S.
12.2(27)SBC	This feature was integrated into Cisco IOS Release 12.2(27)SBC.
12.2(33)SRA	This feature was integrated into Cisco IOS Release 12.2(33)SRA.

Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Contents

•Prerequisites for OSPF Incremental SPF

•Information About OSPF Incremental SPF

•How to Enable OSPF Incremental SPF

•Configuration Examples for OSPF Incremental SPF

•Additional References

•Command Reference

Prerequisites for OSPF Incremental SPF

It is presumed that you have OSPF configured in your network.

Information About OSPF Incremental SPF

Before you enable OSPF Incremental SPF, you should understand the concept described in this section.

•Benefits of OSPF Incremental SPF

Benefits of OSPF Incremental SPF

OSPF uses Dijkstra's SPF algorithm to compute the shortest path tree (SPT). During the computation of the SPT, the shortest path to each node is discovered. The topology tree is used to populate the routing table with routes to IP networks. When changes to a Type-1 or Type-2 link-state advertisement (LSA) occur in an area, the entire SPT is recomputed. In many cases, the entire SPT need not be recomputed because most of the tree remains unchanged. Incremental SPF allows the system to recompute only the affected part of the tree. Recomputing only a portion of the tree rather than the entire tree results in faster OSPF convergence and saves CPU resources. Note that if the change to a Type-1 or Type-2 LSA occurs in the calculating router itself, then the full SPT is performed.

Incremental SPF is scheduled in the same way as the full SPF. Routers enabled with incremental SPF and routers not enabled with incremental SPF can function in the same internetwork.

How to Enable OSPF Incremental SPF

This section contains the following procedure:

•Enabling Incremental SPF

Enabling Incremental SPF

This section describes how to enable incremental SPF.

SUMMARY STEPS

1. enable

2. configure terminal

3. router ospf process-id

4. ispf

5. end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. •Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	router ospf process-id Example: Router(config)# router ospf 1	Configures an OSPF routing process.
Step 4	ispf Example: Router(config-router)# ispf	Enables incremental SPF.
Step 5	end Example: Router(config-router)# end	Exits router configuration mode.

Configuration Examples for OSPF Incremental SPF

This section contains an example of configuring OSPF incremental SPF:

•Incremental SPF: Example

Incremental SPF: Example

This example enables incremental SPF:

router ospf 1

 ispf

Additional References

The following sections provide references related to OSPF Incremental SPF.

Related Documents

Related Topic	Document Title
OSPF commands	Cisco IOS IP Routing Protocols Command Reference

Standards

Standards	Title
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.	—

MIBs

MIBs	MIBs Link
None	To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL: http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

RFCs

RFCs	Title
No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.	—

Technical Assistance

Description	Link
Technical Assistance Center (TAC) home page, containing 30,000 pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.	http://www.cisco.com/public/support/tac/home.shtml

Command Reference

The following command is introduced or modified in the feature or features documented in this module. For information about this command, see the Cisco IOS IP Routing Protocols Command Reference at http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_book.html. For information about all Cisco IOS commands, go to the Command Lookup Tool at http://tools.cisco.com/Support/CLILookup or to the Cisco IOS Master Commands List.

•ispf

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