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This documentation has been moved
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Configuring OSPF
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OSPF ABR Type 3 LSA Filtering
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OSPF Stub Router Advertisement
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OSPF Update Packet-Pacing Configurable Timers
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OSPF Sham-Link Support for MPLS VPN
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OSPF Retransmissions Limit
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OSPF Support for Multi-VRF on CE Routers
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OSPF Forwarding Address Suppression in Translated Type-5 LSAs
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OSPF Inbound Filtering Using Route Maps with a Distribute List
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OSPF Shortest Path First Throttling
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OSPF Support for Fast Hello Packets
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OSPF Incremental SPF
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OSPF Limit on Number of Redistributed Routes
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OSPF Link-State Advertisement (LSA) Throttling
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OSPF Support for Unlimited Software VRFs per Provider Edge (PE) Router
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OSPF Area Transit Capability
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OSPF Per-Interface Link-Local Signaling
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OSPF Link-State Database Overload Protection
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OSPF Enhanced Traffic Statistics for OSPFv2 and OSPFv3
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OSPF MIB Support of RFC 1850 and Latest Extensions
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OSPF: SNMP ifIndex Value for Interface ID in OSPFv2 and OSPFv3 Data Fields
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OSPF RFC 3623 Graceful Restart Helper Mode
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OSPF Mechanism to Exclude Connected IP Prefixes from LSA Advertisements
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OSPFv2 Local RIB
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Feedback
Table Of Contents
OSPF RFC 3623 Graceful Restart Helper Mode
Prerequisites for OSPF RFC 3623 Graceful Restart Helper Mode
Restrictions for OSPF RFC 3623 Graceful Restart Helper Mode
Information About OSPF RFC 3623 Graceful Restart Helper Mode
Cisco NSF Routing and Forwarding Operation
Cisco Express Forwarding for NSF
OSPF Graceful Restart Helper Mode Functionality per RFC 3623
How to Use OSPF RFC 3623 Graceful Restart Helper Mode
Disabling Graceful Restart Helper Mode and Configuring Strict LSA Checking on the Helper Router
Configuration Examples for OSPF RFC 3623 Graceful Restart Helper Mode
Example: Disabling Helper Support for IETF NSF
Feature Information for OSPF RFC 3623 Graceful Restart Helper Mode
OSPF RFC 3623 Graceful Restart Helper Mode
First Published: February 27, 2006Last Updated: October 2, 2009This 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 for OSPF RFC 3623 Graceful Restart Helper Mode" section.
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Contents
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Prerequisites for OSPF RFC 3623 Graceful Restart Helper Mode
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Prerequisites for OSPF RFC 3623 Graceful Restart Helper Mode
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Restrictions for OSPF RFC 3623 Graceful Restart Helper Mode
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Information About OSPF RFC 3623 Graceful Restart Helper Mode
Before configuring the OSPF RFC 3623 Graceful Restart Helper Mode feature, you should understand the following concepts:
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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:
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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
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:
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Helper Mode Exit
The helper router stops performing helper mode for its neighbor when one of the following events occur:
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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
Disabling Graceful Restart Helper Mode and Configuring Strict LSA Checking on the Helper Router
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DETAILED STEPS
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 200Router(config-router)# nsf ietf helper disableAdditional References
The following sections provide references related to the OSPF RFC 3623 Graceful Restart Helper Mode feature.
Related Documents
OSPF commands
OSPF configuration
Cisco NSF feature in Cisco IOS software.
Master list of Cisco IOS commands
Cisco IOS Master Command List, All Releases
Standards
MIBs
None
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
Technical Assistance
Feature Information for OSPF RFC 3623 Graceful Restart Helper Mode
Table 1 lists the release history for this feature.
Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note
Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at 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. (1005R)
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.
© 2011 Cisco Systems, Inc. All rights reserved.
