EIGRP Nonstop Forwarding NSF Awareness
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EIGRP Nonstop Forwarding (NSF) AwarenessLast Updated: July 26, 2012
Nonstop Forwarding (NSF) awareness allows an NSF-aware router to assist NSF-capable and NSF-aware neighbors to continue forwarding packets during a switchover operation or during a well-known failure condition. The EIGRP Nonstop Forwarding Awareness feature allows an NSF-aware router that is running Enhanced Interior Gateway Routing Protocol (EIGRP) to forward packets along routes known to a router performing a switchover operation or in a well-known failure condition. This capability allows the EIGRP peers of the failing router to retain the routing information that it has advertised and to continue using this information until the failed router resumes normal operation and is able to exchange routing information. The peering session is maintained throughout the entire NSF operation.
Finding Feature InformationYour software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and 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 module. 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 EIGRP Nonstop Forwarding Awareness
Restrictions for EIGRP Nonstop Forwarding Awareness
Information About EIGRP Nonstop Forwarding Awareness
Cisco NSF Routing and Forwarding OperationCisco NSF is supported by the BGP, EIGRP, OSPF, and IS-IS protocols for routing and by Cisco Express Forwarding (CEF) for forwarding. Of the routing protocols, BGP, OSPF, and IS-IS have been enhanced with NSF-capability and awareness, which means that routers running these protocols can detect a switchover and take the necessary actions to continue forwarding network traffic and to recover route information from the peer devices. The IS-IS protocol can be configured to use state information that has been synchronized between the active and the standby route processor (RP) to recover route information following a switchover instead of information received from peer devices. In this document, a networking device that is NSF-aware is running NSF-compatible software. A device that is NSF-capable has been configured to support NSF; therefore, the device rebuilds routing information from NSF-aware or NSF-capable neighbors. Each protocol depends on CEF to continue forwarding packets during switchover while the routing protocols rebuild the routing information base (RIB) tables. After the routing protocols have converged, CEF updates the forwarding information base (FIB) table and removes stale route entries. CEF, in turn, updates the line cards with the new FIB information. Cisco Express ForwardingIn a Cisco networking device, CEF provides packet forwarding, a key element of NSF. 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. NSF helps to reduce 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 will maintain the current forwarding information over a switchover; for platforms with forwarding engines, CEF will keep 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 will be able to continue forwarding after a switchover as soon as the interfaces and a data path are available. As the routing protocols start to repopulate the RIB on a prefix-by-prefix basis, the updates in turn cause prefix-by-prefix updates for CEF, which it uses to update the FIB and adjacency databases. Existing and new entries will receive the new version ("epoch") number, indicating that they have been refreshed. The forwarding information is updated on the line cards or forwarding engine 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 routing protocols run only on the active RP, and they receive routing updates from their neighbor routers. Routing protocols do not run on the standby RP. Following a switchover, the routing protocols request that the NSF-aware neighbor devices send state information to help rebuild the routing tables.
EIGRP Nonstop Forwarding AwarenessNSF awareness allows a router that is running EIGRP to assist NSF-capable neighbors to continue forwarding packets during a switchover operation or well-known failure condition. The EIGRP Nonstop Forwarding Awareness feature provides EIGRP with the capability to detect a neighbor that is undergoing an NSF restart event (RP switchover operation) or well-known failure condition, maintain the peering session with this neighbor, retain known routes, and continue to forward packets for these routes. The deployment of EIGRP NSF awareness can minimize the effects of the following:
EIGRP NSF awareness is enabled by default and is transparent to the network operator and EIGRP peers that do not support NSF capabilities.
EIGRP NSF Capable and NSF Aware InteroperationEIGRP NSF capabilities are exchanged by EIGRP peers in hello packets. An NSF-capable router notifies its neighbors that an NSF restart operation has started by setting the restart (RS) bit in a hello packet. When an NSF-aware router receives notification from an NSF-capable neighbor that an NSF-restart operation is in progress, both routers immediately exchange their topology tables. The NSF-aware router sends an end-of-table (EOT) update packet when the transmission of its topology table is complete. The NSF-aware router then performs the following actions to assist the NSF-capable router:
When the switchover operation is complete, the NSF-capable router notifies its neighbors that it has reconverged and has received all of their topology tables by sending an EOT update packet to the assisting routers. The NSF-capable router then returns to normal operation. The NSF-aware router looks for alternate paths (go active) for any routes that are not refreshed by the NSF-capable (restarting) router. The NSF-aware router returns to normal operation. If all paths are refreshed by the NSF-capable router, the NSF-aware router immediately returns to normal operation. Non-NSF Aware EIGRP NeighborsNSF-aware routers are completely compatible with non-NSF aware or non-NSF capable neighbors in an EIGRP network. A non-NSF aware neighbor ignores NSF capabilities and resets the adjacency when they are received. The NSF-capable router drops any queries that are received while converging to minimize the number of transient routes that are sent to neighbors. The NSF-capable router, however, still acknowledges these queries to prevent these neighbors from resetting adjacency.
EIGRP NSF Route-Hold TimersThe route-hold timer is configurable, which allows you to tune network performance and avoid undesired conditions such as "black holing" routes if the switchover operation is lengthy. When the timer expires, the NSF-aware router scans the topology table and discards stale routes, allowing EIGRP peers to find alternate routes instead of waiting during a long switchover operation. The route-hold timer is configured with the timers graceful-restart purge-time router configuration command. The default time period for the route-hold timer is 240 seconds. The configurable range is from 10 to 300 seconds. How to Modify and Maintain EIGRP Nonstop Forwarding Awareness
Adjusting NSF Route-Hold Timers
SUMMARY STEPS
DETAILED STEPS Troubleshooting TipsNeighbor adjacencies are maintained during NSF switchover operations. If adjacencies between NSF-capable and NSF-aware neighbors are being reset too often, the route-hold timers may need to be adjusted. The show ip eigrp neighbors detail command can be used to help determine if the route-hold timer value should be set to a longer time period. The time that adjacency is established with specific neighbors is displayed in the output. This time indicates if adjacencies are being maintained or reset and when the last time that specific neighbors were restarted. Monitoring EIGRP NSF Debug Events and NotificationsPerform the following steps to monitor EIGRP NSF debug events and notifications on an NSF-aware router. The debug eigrp nsf and debug ip eigrp notifications commands are provided together for example purposes only. You do not have to issue these commands together or in the same session as there are differences in the information that is provided. Debugging processes are heavy users of CPU resources. Debug commands should not be used in a production network unless you are troubleshooting a problem. DETAILED STEPS
Verifying the Local Configuration of EIGRP NSF AwarenessPerform the following steps to verify NSF-awareness configuration on a router that is running EIGRP. DETAILED STEPS
Configuration Examples for EIGRP Nonstop Forwarding Awareness
Example Monitoring EIGRP NSF Debug Events and Notifications ConfigurationThe following example output shows that an NSF-aware router has received a restart notification. The NSF-aware router waits for EOT to be sent from the restarting (NSF-capable) neighbor.
Router# debug ip eigrp notifications
*Oct 4 11:39:18.092:EIGRP:NSF:AS2. Rec RS update from 135.100.10.1,
00:00:00. Wait for EOT.
*Oct 4 11:39:18.092:%DUAL-5-NBRCHANGE:IP-EIGRP(0) 2:Neighbor
135.100.10.1 (POS3/0) is up:peer NSF restarted
*Sep 23 18:49:07.578: %DUAL-5-NBRCHANGE: EIGRP-IPv4 100: Neighbor 1.1.2.1 (Ethernet1/0) is resync: peer graceful-restart
Example Verifying Local Configuration of EIGRP NSF AwarenessThe following is example output from the show ip protocols command. The output from this command can be used to verify the local configuration of EIGRP NSF awareness. The output shows that the router is NSF-aware and that the route-hold timer is set to 240 seconds, which is the default value.
Router# show ip protocols
*** IP Routing is NSF aware ***
Routing Protocol is "eigrp 101"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 101
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.4.9.0/24
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170
Additional ReferencesRelated Documents
MIBsTechnical Assistance
Feature Information for EIGRP Nonstop Forwarding AwarenessThe 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.
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. © 2012 Cisco Systems, Inc. All rights reserved.
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