IP Routing: OSPF Configuration Guide, Cisco IOS Release 12.4T
OSPF RFC 3623 Graceful Restart Helper Mode
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OSPF RFC 3623 Graceful Restart Helper Mode

OSPF RFC 3623 Graceful Restart Helper Mode

Last Updated: October 27, 2011

This document focuses on Nonstop Forwarding (NSF) helper mode for OSPFv2 in Cisco IOS software, using IETF standardized graceful restart helper mode functionality as described in RFC 3623, Graceful OSPF Restart. Graceful restart helper mode functionality, which is enabled by default, is useful for multiplatform network environments where helper mode routers on some platforms can assist restarting routers on mixed platforms that support OSPF graceful restart mode as well as helper mode.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Prerequisites for OSPF RFC 3623 Graceful Restart Helper Mode

  • OSPF must be configured on the router.

Restrictions for OSPF RFC 3623 Graceful Restart Helper Mode

  • IETF Graceful Restart is not supported over sham-links.

Information About OSPF RFC 3623 Graceful Restart Helper Mode

Cisco NSF Routing and Forwarding Operation

Prior to RFC 3623 Cisco implemented the Cisco proprietary NSF referred to as Cisco NSF. Users can configure either Cisco or RFC 3623 IETF NSF, depending on which versions are supported by the Cisco IOS software version running on the network. If the software version supports both types of NSF, you need to configure all routers with the same type of NSF.

Cisco NSF is supported by the BGP, EIGRP, OSPF, and IS-IS protocols for routing and by Cisco Express Forwarding (CEF) for forwarding. The BGP, EIGRP, OSPF, and IS-IS routing protocols have been enhanced with NSF capability and awareness, which means that routers that run these protocols can detect a switchover and take the necessary actions to continue forwarding network traffic and to recover route information from the neighbor routers.

In this document, a networking device is said to be NSF-aware if it is running NSF-compatible software. A device is said to be NSF-capable if it has been configured to support NSF; therefore, it would rebuild routing information from NSF-aware or NSF-capable neighbors. The NSF router mode of operation common to the Cisco and IETF NSF implementations is as follows:

  • Restarting Mode--Also known as IETF NSF-restarting mode or graceful-restarting mode. In this mode, the OSPF router process is performing non-stop forwarding recovery because of an RP switchover; this may result from an RP crash or a software upgrade on the active RP.
  • Helper Mode--Also known as IETF NSF-awareness. In this mode, the neighboring router is restarting and helping in the NSF recovery.

For more information about OSPF RFC 3623 Graceful Restart, see http://www.ietf.org/rfc/rfc3623.txt .

Cisco Express Forwarding for NSF

A key element of NSF is packet forwarding. The OSPF protocol depends on CEF to continue forwarding packets during switchover while the routing protocols rebuild the Routing Information Base (RIB) tables. Once OSPF has converged, CEF updates the Forwarding Information Base (FIB) table and removes stale route entries. CEF then updates the line cards with the new FIB information. CEF maintains the FIB and uses the FIB information that was current at the time of a switchover to continue forwarding packets during the switchover. This feature reduces traffic interruption during the switchover.

During normal NSF operation, CEF on the active RP synchronizes its current FIB and adjacency databases with the FIB and adjacency databases on the standby RP. Upon switchover of the active RP, the standby RP initially has FIB and adjacency databases that are mirror images of those that were current on the active RP. For platforms with intelligent line cards, the line cards maintain the current forwarding information over a switchover; for platforms with forwarding engines, CEF keeps the forwarding engine on the standby RP current with changes that are sent to it by CEF on the active RP. In this way, the line cards or forwarding engines can continue forwarding after a switchover as soon as the interfaces and a data path are available.

As the OSPF routing protocol starts to repopulate the RIB on a prefix-by-prefix basis, the updates in turn cause prefix-by-prefix updates that CEF uses to update the FIB and adjacency databases. Existing and new entries receive the new version number, indicating that they have been refreshed. The forwarding information is updated on the line cards or forwarding engines during convergence. The RP signals when the RIB has converged. The software removes all FIB and adjacency entries that have an epoch older than the current switchover epoch. The FIB now represents the newest routing protocol forwarding information.

The OSPF routing protocol runs on only the active RP, and OSPF receives routing updates from OSPF neighbor routers. The OSPF routing protocol does not run on the standby RP. Following a switchover, OSPF requests that the NSF-aware neighbor devices send state information to help rebuild the routing tables.


Note


For NSF operation, OSPF depends on CEF to continue forwarding packets while OSPF rebuilds the routing information.

OSPF Graceful Restart Helper Mode Functionality per RFC 3623

Helper Mode Initiation

When a neighbor router that is on the same network segment as the restarting router receives a grace-LSA from the restarting router, the neighbor enters helper mode as long as the following criteria are met:

  • The neighbor must have a full adjacency with the restarting router over the associated network segment.
  • There have been no changes to the link-state database since the restarting router began restarting.
  • The grace period has not yet expired.
  • Local policy allows the neighbor router to act as a helper router.
  • The neighbor router must not be in its own graceful restart process.
  • Helper mode for this router has not been disabled by the network administrator.

Helper Mode Exit

The helper router stops performing helper mode for its neighbor when one of the following events occur:

  • The grace-LSA that was originated by the restarting router is flushed, to signify that the restarting router has exited the graceful restart process successfully.
  • The grace period of the grace-LSA expires.
  • A change in link-state database contents indicates a network topology change, forcing the termination of the graceful restart process.

For complete information about graceful restart functionality, see RFC 3623 at http://www.ietf.org/rfc/rfc3623.txt .

The OSPF RFC 3623 Graceful Restart Helper Mode feature is enabled by default. Disabling this feature is not recommended because the disabled neighbor will detect the lost adjacency and the graceful restart process will be terminated on the restarting neighbor router.

The strict LSA checking feature allows a helper router to terminate the graceful restart process if it detects a changed LSA that would cause flooding during the graceful restart process. Strict LSA checking is disabled by default. You can enable strict LSA checking when there is a change to an LSA that would be flooded to the restarting router. You can configure strict LSA checking on both NSF-aware and NSF-capable routers; however, it becomes effective only when the router is in helper mode.

How to Use OSPF RFC 3623 Graceful Restart Helper Mode

Configuring Strict LSA Checking on the Helper Router

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    router ospf process-id [vrf vpn-name]

4.    nsf ietf helper disable

5.    nsf ietf helper strict-lsa-checking

6.    end

7.    show ip ospf [process-id]


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 [vrf vpn-name]


Example:

Router(config)# router ospf 454

 

Configures an Open Shortest Path First (OSPF) routing process and enters router configuration mode.

 
Step 4
nsf ietf helper disable


Example:

Router(config-router)# nsf ietf helper disable

 

Disables helper mode for IETF NSF.

 
Step 5
nsf ietf helper strict-lsa-checking


Example:

Router(config-router)# nsf ietf strict-lsa-checking

 

Enables strict LSA checking on an NSF-aware (helper) router.

 
Step 6
end


Example:

Router(config-router)# end

 

Returns to privileged EXEC mode.

 
Step 7
show ip ospf [process-id]


Example:

Router# show ip ospf 454

 

Displays general information about OSPF routing processes and whether helper mode or strict LSA checking is enabled for the NSF-aware (helper) router.

 

Configuration Examples for OSPF RFC 3623 Graceful Restart Helper Mode

Example Disabling Helper Support for IETF NSF

The following configuration example disables helper support for OSPF NSF.

Router(config)# router ospf 200
Router(config-router)# nsf ietf helper disable

Additional References

The following sections provide references related to the OSPF RFC 3623 Graceful Restart Helper Mode feature.

Related Documents

Related Topic

Document Title

OSPF commands

Cisco IOS IP Routing: OSPF Command Reference

OSPF configuration

"Configuring OSPF"

Cisco NSF feature in Cisco IOS software.

"Cisco Nonstop Forwarding"

Master list of Cisco IOS commands

Cisco IOS Master Command List, All Releases

Standards

Standard

Title

None

--

MIBs

MIB

MIBs Link

None

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFC

Title

RFC 2328

OSPF Version 2

RFC 3623

Graceful OSPF Restart

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Feature Information for OSPF RFC 3623 Graceful Restart Helper Mode

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1 Feature Information for OSPF RFC 3623 Graceful Restart Helper Mode

Feature Name

Releases

Feature Information

OSPF RFC 3623 Graceful Restart Helper Mode

12.4(6)T

This document focuses on NSF for OSPFv2 in Cisco IOS software, using IETF standardized graceful restart functionality as described in RFC 3623.

The following commands were introduced or modified: nsf cisco helper disable, nsf ietf helper disable, nsf ietf helper strict-lsa-checking.

Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S. and other countries. To view a list of Cisco trademarks, go to this URL: www.cisco.com/go/trademarks. Third-party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1110R)

Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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