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Table Of Contents
Configuring Nonstop Forwarding
Prerequisites for Configuring Nonstop Forwarding
Restrictions for Configuring Nonstop Forwarding
Information About Configuring Nonstop Forwarding
Cisco NSF Routing and Forwarding
Cisco Express Forwarding and NSF
IPv6 support for NSF Operations
Nonstop Forwarding and Graceful Restart for MP-BGP IPv6 Address Family
Nonstop Forwarding for IPv6 RIP
Nonstop Forwarding for Static Routes
How to Configure Nonstop Forwarding
Configuring and Verifying BGP NSF
Configuring and Verifying EIGRP NSF
Configuring Cisco NSF for OSPF
Configuring and Verifying IS-IS NSF
Troubleshooting Nonstop Forwarding
Configuration Examples for Configuring Nonstop Forwarding
Feature Information for Configuring Nonstop Forwarding
Configuring Nonstop Forwarding
First Published: July 22, 2002Last Updated: November 22, 2010This module describes how to configure Nonstop Forwarding (NSF) in Cisco software to minimize the amount of time a network is unavailable to its users following a switchover. The main objective of NSF is to continue forwarding IP packets following a Route Processor (RP) switchover. NSF is supported by the BGP, EIGRP, IPv6, IS-IS, and OSPF protocols for routing and by CEF for forwarding.
The following terms are used throughout this document:
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NSF-aware device—A device that is running NSF-compatible software
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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 Configuring Nonstop Forwarding" section.
Use Cisco Feature Navigator to find information about platform support and Cisco 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 Configuring Nonstop Forwarding
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Restrictions for Configuring Nonstop Forwarding
General Restrictions
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BGP NSF Restrictions
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EIGRP NSF Restrictions
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OSPF NSF Restrictions
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Information About Configuring Nonstop Forwarding
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Nonstop Forwarding
Note
NSF works with the SSO feature in Cisco software to minimize the amount of time a network is unavailable to its users following a switchover. The main objective of NSF is to continue forwarding IP packets following an RP switchover.
Usually, when a networking device restarts, all routing peers of that device detect that the device went down and then came back up. This transition results in what is called a routing flap, which could spread across multiple routing domains. Routing flaps caused by routing restarts create routing instabilities, which are detrimental to the overall network performance. NSF helps to suppress routing flaps in SSO-enabled devices, thus reducing network instability.
NSF allows for the forwarding of data packets to continue along known routes while the routing protocol information is being restored following a switchover. With NSF, peer networking devices do not experience routing flaps. Data traffic is forwarded through intelligent line cards or dual forwarding processors (FPs) while the standby RP assumes control from the failed active RP during a switchover. The ability of line cards and FPs to remain up through a switchover and to be kept current with the Forwarding Information Base (FIB) on the active RP is key to NSF operation.
The NSF feature provides the following benefits:
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NSF always runs together with SSO. SSO supported protocols and applications must be high-availability (HA)-aware. A feature or protocol is HA-aware if it maintains, either partially or completely, undisturbed operation through an RP switchover. For some HA-aware protocols and applications, state information is synchronized from the active to the standby processor. For Cisco NSF, enhancements to the routing protocols (CEF; OSPF; BGP; and IS-IS) have been made to support the HA features in SSO.
Cisco NSF Routing and Forwarding
Cisco NSF is supported by the BGP, EIGRP, IPv6, IS-IS, and OSPF protocols for routing and by CEF for forwarding. Of the routing protocols, BGP, EIGRP, IPv6, IS-IS, and OSPF have been enhanced with NSF-capability and awareness, which means that devices 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 RP to recover route information following a switchover instead of information received from peer devices.
Each protocol depends on CEF to continue forwarding packets during switchover while the routing protocols rebuild the Routing Information Base (RIB) tables. Once the routing protocols have converged, CEF updates the FIB table and removes stale route entries. CEF, in turn, updates the line cards with the new FIB information.
Cisco Express Forwarding and NSF
A key element of NSF is packet forwarding. In a Cisco networking device, packet forwarding is provided by CEF. CEF maintains the FIB, and uses the FIB information that was current at the time of the switchover to continue forwarding packets during a 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 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 to 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 devices. 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. Alternately, the IS-IS protocol can be configured to synchronize state information from the active to the standby RP to help rebuild the routing table on the NSF-capable device in environments where neighbor devices are not NSF-aware.
For NSF operation, the routing protocols depend on CEF to continue forwarding packets while the routing protocols rebuild the routing information. The CEF NSF feature operates by default while the networking device is running in SSO mode. No configuration is necessary.
BGP NSF Operations
When a NSF-capable device begins a BGP session with a BGP peer, it sends an OPEN message to the peer. Included in the message is a declaration that the NSF-capable device has "graceful restart capability." Graceful restart is the mechanism by which BGP routing peers avoid a routing flap following a switchover. If the BGP peer has received this capability, it is aware that the device sending the message is NSF-capable. Both the NSF-capable device and its BGP peers need to exchange the graceful restart capability in their OPEN messages, at the time of session establishment. If both the peers do not exchange the graceful restart capability, the session will not be graceful restart capable.
If the BGP session is lost during the RP switchover, the NSF-aware BGP peer marks all the routes associated with the NSF-capable device as stale; however, it continues to use these routes to make forwarding decisions for a set period of time. This functionality means that no packets are lost while the newly active RP is waiting for convergence of the routing information with the BGP peers.
After an RP switchover occurs, the NSF-capable device reestablishes the session with the BGP peer. In establishing the new session, it sends a new graceful restart message that identifies the NSF-capable device as having restarted.
At this point, the routing information is exchanged between the two BGP peers. Once this exchange is complete, the NSF-capable device uses the routing information to update the RIB and the FIB with the new forwarding information. The NSF-aware device uses the network information to remove stale routes from its BGP table. Following that, the BGP protocol is fully converged.
If a BGP peer does not support the graceful restart capability, it will ignore the graceful-restart capability in an OPEN message but will establish a BGP session with the NSF-capable device. This function will allow interoperability with non-NSF-aware BGP peers (and without NSF functionality), but the BGP session with non-NSF-aware BGP peers will not be graceful restart-capable.
BGP support in NSF requires that neighbor networking devices be NSF-aware; that is, the devices must have the graceful restart capability and advertise that capability in their OPEN message during session establishment. If an NSF-capable device discovers that a particular BGP neighbor does not have graceful restart capability, it will not establish an NSF-capable session with that neighbor. All other neighbors that have graceful restart capability will continue to have NSF-capable sessions with this NSF-capable networking device.
EIGRP NSF Operations
EIGRP NSF capabilities are exchanged by EIGRP peers in hello packets. The NSF-capable device notifies its neighbors that an NSF restart operation has started by setting the restart (RS) bit in a hello packet. When an NSF-aware device receives notification from an NSF-capable neighbor that an NSF-restart operation is in progress, the NSF-capable and NSF-aware devices immediately exchange their topology tables. The NSF-aware device sends an end-of-table (EOT) update packet when the transmission of its topology table is complete. The NSF-aware device then performs the following actions to assist the NSF-capable device:
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When the switchover operation is complete, the NSF-capable device notifies its neighbors that it has reconverged and has received all of their topology tables by sending an EOT update packet to the assisting devices. The NSF-capable device then returns to normal operation. The NSF-aware device will look for alternate paths (go active) for any routes that are not refreshed by the NSF-capable (restarting device). The NSF-aware device will then return to normal operation. If all paths are refreshed by the NSF-capable device, the NSF-aware device will immediately return to normal operation.
NSF-aware devices are completely compatible with non-NSF aware or -capable neighbors in an EIGRP network. A non-NSF-aware neighbor will ignore NSF capabilities and reset adjacencies and otherwise maintain the peering sessions normally.
IPv6 support for NSF Operations
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Nonstop Forwarding and Graceful Restart for MP-BGP IPv6 Address Family
The graceful restart capability is supported for IPv6 BGP unicast, multicast, and VPNv6 address families, enabling NSF functionality for BGP IPv6. The BGP graceful restart capability allows the BGP routing table to be recovered from peers without keeping the TCP state.
NSF continues forwarding packets while routing protocols converge, therefore avoiding a route flap on switchover. Forwarding is maintained by synchronizing the FIB between the active and standby RP. On switchover, forwarding is maintained using the FIB. The RIB is not kept synchronized; therefore, the RIB is empty on switchover. The RIB is repopulated by the routing protocols and subsequently informs the FIB about RIB convergence by using the NSF_RIB_CONVERGED registry call. The FIB tables are updated from the RIB, removing any stale entries. The RIB starts a fail-safe timer during RP switchover, in case the routing protocols fail to notify the RIB of convergence.
The Cisco BGP address family identifier (AFI) model is modular and scalable, and supports multiple AFIs and subsequent address family identifier (SAFI) configurations.
Nonstop Forwarding for IPv6 RIP
RIP registers as an IPv6 NSF client. Doing so has the benefit of using RIP routes installed in the Cisco Express Forwarding table until RIP has converged on the standby.
Nonstop Forwarding for Static Routes
Cisco NSF supports IPv6 static routes.
IS-IS NSF Operations
When an IS-IS NSF-capable device performs an RP switchover, it must perform two tasks in order to resynchronize its Link State Database with its IS-IS neighbors. First, it must relearn the available IS-IS neighbors on the network without causing a reset of the neighbor relationship. Second, it must reacquire the contents of the Link State Database for the network.
The IS-IS NSF feature offers two options when configuring NSF:
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If neighbor devices on a network segment are NSF-aware, meaning that neighbor devices are running a software version that supports the IETF Internet draft for device restartability, they will assist an IETF NSF device that is restarting. With IETF, neighbor devices 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.
If you configure IETF on the networking device, but neighbor devices are not IETF-compatible, NSF will abort following a switchover.
If the neighbor devices 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 standby RP. A benefit of Cisco configuration is that it does not rely on NSF-aware neighbors.
IETF IS-IS Configuration
With the IETF IS-IS configuration, the NSF-capable device sends IS-IS NSF restart requests to neighboring NSF-aware devices as quickly as possible after an RP switchover. Neighbor networking devices recognize this restart request as a cue that the neighbor relationship with this device should not be reset, but that they should initiate database resynchronization with the restarting device. As the restarting device receives restart request responses from devices on the network, it can begin to rebuild its neighbor list.
Once this exchange is complete, the NSF-capable device uses the link-state information to remove stale routes, update the RIB, and update the FIB with the new forwarding information. IS-IS is then fully converged.
The switchover from one RP to the other happens within seconds. IS-IS reestablishes its routing table and resynchronizes with the network within a few additional seconds. At this point, IS-IS waits for a specified interval before it will attempt a second NSF restart. During this time, the new standby RP will boot up and synchronize its configuration with the active RP. The IS-IS NSF operation waits for a specified interval to ensure that connections are stable before attempting another restart of IS-IS NSF. This functionality prevents IS-IS from attempting back-to-back NSF restarts with stale information.
Cisco IS-IS Configuration
With the Cisco configuration option, full adjacency and link-state packet (LSP) information is saved, or "checkpointed," to the standby RP. Following a switchover, the newly active RP maintains its adjacencies using the checkpointed data, and can quickly rebuild its routing tables.
The switchover from one RP to the other happens within seconds. IS-IS reestablishes its routing table and resynchronizes with the network within a few additional seconds. At this point, IS-IS waits for a specified interval before it will attempt a second NSF restart. During this time, the new standby RP will boot up and synchronize its configuration with the active RP. Once this synchronization is completed, IS-IS adjacency and LSP data is checkpointed to the standby RP; however, a new NSF restart will not be attempted by IS-IS until the interval time expires. This functionality prevents IS-IS from attempting back-to-back NSF restarts. IS-IS NSF provides a command to extend the wait time for interfaces that, for whatever reason, do not come up in a timely fashion.
Following a switchover, Cisco IS-IS NSF has complete neighbor adjacency and LSP information; however, it must wait for all interfaces that had adjacencies prior to the switchover to come up. If an interface does not come up within the allocated interface wait time, the routes learned from these neighbor devices are not considered in routing table recalculation.
OSPF NSF Operations
Before an OSPF NSF-capable device can perform an RP switchover, it must relearn the available OSPF neighbors on the network, without resetting the neighbor relationship, and it must reacquire the contents of the Link State Database for the network.
To do this, the NSF-capable device sends an OSPF NSF signal to neighboring NSF-aware devices to notify the devices that the neighbor relationship with the sending device must not be reset. Then the NSF-capable device uses the signals it receives from other devices on the network to rebuild its neighbor list.
Next, the NSF-capable device resynchronizes its database with all of the NSF-aware neighbors on its list. After all of the neighbors exchange routing information, 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.
Prior to RFC 3623, Cisco implemented the proprietary Cisco NSF. The RFC 3623 Graceful OSPF Restart feature supports IETF NSF for OSPF processes in multivendor networks. The NSF device modes of operation common to the Cisco and IETF NSF implementations are as follows:
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The strict LSA checking feature allows a helper device 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 device.
How to Configure Nonstop Forwarding
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Configuring and Verifying BGP NSF
Repeat this procedure on each peer device.
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DETAILED STEPS
Configuring and Verifying EIGRP NSF
Repeat this task on each peer device.
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Configuring OSPF NSF
Perform only one of the following tasks to configure OSPF NSF:
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Configuring Cisco NSF for OSPF
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Configuring IETF NSF for OSPF
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Configuring and Verifying IS-IS NSF
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Troubleshooting Nonstop Forwarding
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Configuration Examples for Configuring Nonstop Forwarding
Example: NSF-Capable CEF
The CEF NSF feature operates by default while the networking device is running in SSO mode. No configuration is necessary. The following sample output shows that CEF is NSF capable:
Router# show cef stateCEF Status [RP]CEF enabled/runningdCEF enabled/runningCEF switching enabled/runningCEF default capabilities:Always FIB switching: yesDefault CEF switching: yesDefault dCEF switching: yesUpdate HWIDB counters: noDrop multicast packets: noCEF NSF capable: yesIPC delayed func on SSO: noRRP state:I am standby RRP: noMy logical slot: 0RF PeerComm: noExample: BGP NSF
The following partial output shows the BGP configuration on the SSO-enabled device:
Router# show running-configrouter bgp 120bgp graceful-restartneighbor 10.2.2.2 remote-as 300The following sample output shows that the graceful restart function is both advertised and received and that the address families have the graceful restart capability. If no address families were listed, then BGP NSF will not occur.
Router# show ip bgp neighbors 192.168.2.2BGP neighbor is 192.168.2.2, remote AS YY, external linkBGP version 4, remote router ID 192.168.2.2BGP state = Established, up for 00:01:18Last read 00:00:17, hold time is 180, keepalive interval is 60 secondsNeighbor capabilities:Route refresh:advertised and received(new)Address family IPv4 Unicast:advertised and receivedAddress family IPv4 Multicast:advertised and receivedGraceful Restart Capabilty:advertised and receivedRemote Restart timer is 120 secondsAddress families preserved by peer:IPv4 Unicast, IPv4 MulticastReceived 1539 messages, 0 notifications, 0 in queueSent 1544 messages, 0 notifications, 0 in queueDefault minimum time between advertisement runs is 30 secondsExample: EIGRP NSF
The following sample output shows that EIGRP NSF support is present in the installed software image.
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Router# show ip protocolsRouting Protocol is "eigrp 100"Outgoing update filter list for all interfaces is not setIncoming update filter list for all interfaces is not setDefault networks flagged in outgoing updatesDefault networks accepted from incoming updatesEIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0EIGRP maximum hopcount 100EIGRP maximum metric variance 1Redistributing: eigrp 100EIGRP NSF-aware route hold timer is 240sEIGRP NSF enabledNSF signal timer is 20sNSF converge timer is 120sAutomatic network summarization is in effectMaximum path: 4Routing for Networks:10.4.9.0/24Routing Information Sources:Gateway Distance Last UpdateDistance: internal 90 external 170Example: OSPF and Cisco NSF
The following output from the show ip ospf nsf command shows that NSF is enabled for OSPF process 400. NSF helper mode is enabled by default on routers running NSF-compatible software. Note that in this configuration, IETF helper mode is disabled for process 400.
Router> show ip ospf nsfRouting Process "ospf 400"Non-Stop Forwarding enabledIETF NSF helper support disabledCisco NSF helper support enabledOSPF restart state is NO_RESTARTHandle 2162698, Router ID 192.0.2.155, checkpoint Router ID 0.0.0.0Config wait timer interval 10, timer not runningDbase wait timer interval 120, timer not runningExample: OSPF and IETF NSF
The following output from the show ip ospf nsf command shows that NSF is enabled for OSPF process 500. NSF helper mode is enabled by default on routers running NSF-compatible software. Note that in this configuration, Cisco helper mode is disabled.
Router> show ip ospf nsfRouting Process "ospf 500"Non-Stop Forwarding enabledIETF NSF helper support enabledCisco NSF helper support disabledOSPF restart state is NO_RESTARTHandle 1786466333, Router ID 192.0.2.2, checkpoint Router ID 0.0.0.0Config wait timer interval 10, timer not runningDbase wait timer interval 120, timer not runningExample: IS-IS NSF
The following partial output shows that this device uses the Cisco implementation of IS-IS NSF. The display will show either Cisco IS-IS or IETF IS-IS configuration.
Router# show running-configrouter isisnsf ciscoIn a Cisco NSF configuration, the display output will be different on the active and standby RPs.
The following sample output on the active RP shows that Cisco NSF is enabled on the device:
Router# show isis nsfthe device (NSF restart enabled):
Router# show isis nsfNSF is ENABLED, mode 'cisco'RP is ACTIVE, standby ready, bulk sync completeNSF interval timer expired (NSF restart enabled)Checkpointing enabled, no errorsLocal state:ACTIVE, Peer state:STANDBY HOT, Mode:SSOThe following sample output on the standby RP shows that NSF is enabled on the device (NSF restart enabled):
Router# show isis nsfNSF enabled, mode 'cisco'RP is STANDBY, chkpt msg receive count:ADJ 2, LSP 7NSF interval timer notification received (NSF restart enabled)Checkpointing enabled, no errorsLocal state:STANDBY HOT, Peer state:ACTIVE, Mode:SSOThe following sample output shows that IETF NSF is configured for the IS-IS networking device:
Router# show isis nsfNSF is ENABLED, mode IETFNSF pdb state:InactiveNSF L1 active interfaces:0NSF L1 active LSPs:0NSF interfaces awaiting L1 CSNP:0Awaiting L1 LSPs:NSF L2 active interfaces:0NSF L2 active LSPs:0NSF interfaces awaiting L2 CSNP:0Awaiting L2 LSPs:Interface:Serial3/0/2NSF L1 Restart state:RunningNSF p2p Restart retransmissions:0Maximum L1 NSF Restart retransmissions:3L1 NSF ACK requested:FALSEL1 NSF CSNP requested:FALSENSF L2 Restart state:RunningNSF p2p Restart retransmissions:0Maximum L2 NSF Restart retransmissions:3L2 NSF ACK requested:FALSEInterface:GigabitEthernet2/0/0NSF L1 Restart state:RunningNSF L1 Restart retransmissions:0Maximum L1 NSF Restart retransmissions:3L1 NSF ACK requested:FALSEL1 NSF CSNP requested:FALSENSF L2 Restart state:RunningNSF L2 Restart retransmissions:0Maximum L2 NSF Restart retransmissions:3L2 NSF ACK requested:FALSEL2 NSF CSNP requested:FALSEInterface:Loopback1NSF L1 Restart state:RunningNSF L1 Restart retransmissions:0Maximum L1 NSF Restart retransmissions:3L1 NSF ACK requested:FALSEL1 NSF CSNP requested:FALSENSF L2 Restart state:RunningNSF L2 Restart retransmissions:0Maximum L2 NSF Restart retransmissions:3L2 NSF ACK requested:FALSEL2 NSF CSNP requested:FALSEAdditional References
Related Documents
Cisco IOS commands
Cisco debug commands
Cisco IOS High Availability commands
BGP support for NSF/SSO
BGP Support for Nonstop Routing (NSR) with Stateful Switchover (SSO)
EIGRP NSF awareness
IPv6 BGP Graceful Restart
"Implementing Multiprotocol BGP for IPv6," Cisco IOS XE IPv6 Configuration Guide
IPv6 RIP
"Implementing RIP for IPv6," Cisco IOS XE IPv6 Configuration Guide
IPv6 static routes
"Implementing Static Routes for IPv6," Cisco IOS XE IPv6 Configuration Guide
NSF/SSO—MPLS VPN
"Configuring NSF/SSO—MPLS VPN" module in the MPLS Configuration Guide
Standards
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
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MIBs
RFCs
RFC 3623
Graceful OSPF Restart
RFC 3847
Restart Signaling for Intermediate System to Intermediate System (IS-IS)
RFC 4781
Graceful Restart Mechanism for BGP
Technical Assistance
Feature Information for Configuring Nonstop Forwarding
Table 1 lists the features in this module and provides links to specific configuration information.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which 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. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.
© 2002-2010 Cisco Systems, Inc. All rights reserved.
