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Table Of Contents
Prerequisites for Cisco Nonstop Forwarding
Restrictions for Cisco Nonstop Forwarding
Information About Cisco Nonstop Forwarding
Cisco NSF Routing and Forwarding Operation
Configuring and Verifying CEF NSF
Configuring and Verifying BGP NSF
Configuring and Verifying EIGRP NSF
Configuring and Verifying OSPF NSF
Configuring and Verifying IS-IS NSF
Verifying that CEF Is NSF Capable: Example
Configuring BGP NSF Neighbor Device: Example
Configuring EIGRP NSF Converge Timer: Example
Configuring EIGRP NSF Route-Hold Timer: Example
Configuring EIGRP NSF Signal Timer: Example
Disabling EIGRP NSF Support: Example
Configuring IS-IS NSF: Example
Feature Information for Cisco Nonstop Forwarding
Cisco Nonstop Forwarding
First Published: July 22, 2002Last Updated: December 4, 2006Cisco Nonstop Forwarding (NSF) works with the Stateful Switchover (SSO) feature in Cisco IOS software. NSF works with SSO to minimize the amount of time a network is unavailable to its users following a switchover. The main objective of Cisco NSF is to continue forwarding IP packets following a Route Processor (RP) switchover.
Finding Feature Information in This Module
Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for Cisco Nonstop Forwarding" section.
Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images
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.
Note
Throughout this document, the term "Route Processor" is used to describe the route processing engine on all networking devices, regardless of the platform designation, unless otherwise noted. For example, on the Cisco 10000 series Internet router the RP is called the Performance Routing Engine (PRE), on the Cisco 12000 series Internet router the RP is called the Gigabit Route Processor (GRP), and on the Cisco 7500 series router the RP is called the Route Switch Processor (RSP).
Contents
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Prerequisites for Cisco Nonstop Forwarding
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Restrictions for Cisco Nonstop Forwarding
General Restrictions
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BGP NSF
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EIGRP NSF
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OSPF NSF
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IS-IS NSF
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Cisco 7200 Series Router
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The Cisco 7206 does support NSF and can operate in a peer role with a Cisco 7500, 10000, or 12000 series router running Cisco IOS Release 12.0(23)S. With NSF enabled, an RP switchover on the Cisco 7500, 10000, or 12000 series router peer should not cause a loss of PPP, ATM, high-level data link control (HDLC), or Frame Relay sessions, or a loss of any OSPF, BGP, or IS-IS adjacencies established between the Cisco 7200 and the peer.
Information About Cisco Nonstop Forwarding
Cisco NSF works with the Stateful Switchover (SSO) feature in Cisco IOS software. NSF works with SSO to minimize the amount of time a network is unavailable to its users following a switchover. The main objective of Cisco NSF is to continue forwarding IP packets following a route processor (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. Cisco NSF helps to suppress routing flaps in SSO-enabled devices, thus reducing network instability.
Cisco 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 Cisco 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 Cisco NSF operation.
NSF Dependency on SSO
Cisco NSF always runs together with SSO. This section provides some background information on the SSO feature.
In specific Cisco networking devices that support dual RPs, SSO establishes one of the RPs as the active processor while the other RP is designated as the standby processor, and then synchronizes information between them. A switchover from the active to the standby processor occurs when the active RP fails, is removed from the networking device, or is manually taken down for maintenance.
In networking devices running SSO, both RPs must be running the same configuration so that the standby RP is always ready to assume control following a fault on the active RP. The configuration information is synchronized from the active RP to the standby RP at startup and whenever changes to the active RP configuration occur. Following an initial synchronization between the two processors, SSO maintains RP state information between them, including forwarding information.
During switchover, system control and routing protocol execution is transferred from the active processor to the standby processor. The time required by the device to switch over from the active to the standby processor ranges from just a few seconds to approximately 30 seconds, depending on the platform.
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 (Cisco Express Forwarding, or CEF; Open Shortest Path First, or OSPF; Border Gateway Protocol, or BGP; and Intermediate System-to-Intermediate System, or IS-IS) have been made to support the HA features in SSO.
For more information on SSO, see the "How to Implement Cisco NSF" section.
Cisco NSF Routing and Forwarding Operation
Cisco 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, EIGRP, 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 RP to recover route information following a switchover instead of information received from peer devices.
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.
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.
Table 1 lists the routing protocol and CEF support in Cisco NSF.
Table 1 Routing Protocol and CEF Support in Cisco NSF
BGP
Cisco 7200
Yes1
Yes1
Yes1
No2
No
No
Cisco 7304
No
No
No
No
Yes
No
Cisco 7500
Yes
Yes
Yes
Yes
No
No
Cisco 7600
No
No
No
No
No
Yes
Cisco 10000
Yes
Yes
Yes
No
Yes
No
Cisco 12000
Yes
Yes
Yes
No
No
No
OSPF
Cisco 7200
Yes1
Yes1
Yes1
No2
No
No
Cisco 7304
No
No
No
No
Yes
No
Cisco 7500
Yes
Yes
Yes
Yes
No
No
Cisco 7600
No
No
No
No
No
Yes
Cisco 10000
Yes
Yes
Yes
No
No
No
Cisco 12000
Yes
Yes
Yes
No
No
No
IS-IS
Cisco 7200
Yes1
Yes1
Yes1
No2
No
No
Cisco 7304
No
No
No
No
Yes
No
Cisco 7500
Yes
Yes
Yes
Yes
No
No
Cisco 7600
No
No
No
No
No
Yes
Cisco 10000
Yes
Yes
Yes
No
Yes
No
Cisco 12000
Yes
Yes
Yes
No
No
No
CEF
Cisco 72003
—
—
—
—
—
—
Cisco 7304
No
No
No
No
Yes
No
Cisco 7500
Yes
Yes
Yes
Yes
No
No
Cisco 7600
No
No
No
No
No
Yes
Cisco 10000
Yes
Yes
Yes
No
No
No
Cisco 12000
Yes
Yes
Yes
No
No
No
EIGRP
Cisco 7200
No
No
No
Yes2
No
No
Cisco 7304
No
No
No
No
Yes
No
Cisco 7500
No
No
No
Yes
No
No
Cisco 7600
No
No
No
No
No
Yes
Cisco 10000
No
No
No
No
No
No
Cisco 12000
No
No
No
No
No
No
1 The Cisco 7200 is a single-route processor system and cannot maintain its forwarding table in the event of a route processor failure. It cannot perform nonstop forwarding of packets. However, it supports the NSF protocol extensions for BGP, EIGRP, OSPF, and ISIS. Therefore, it can peer with NSF-capable routers and facilitate the resynchronization of routing information with such routers.
2 The Cisco 7200 is NSF-aware in the Cisco IOS Release 12.2(18)S.
3 The Cisco 7200 is a single processor device and does not support SSO; therefore, CEF support for NSF does not apply.
Cisco Express Forwarding
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.
Routing Protocols
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. 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.
Note
BGP Operation
When a NSF-capable router 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 router and its BGP peer(s) 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 router 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 router reestablishes the session with the BGP peer. In establishing the new session, it sends a new graceful restart message that identifies the NSF-capable router 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.
Note
EIGRP Operation
EIGRP NSF capabilities are exchanged by EIGRP peers in hello packets. The 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, the NSF-capable and NSF-aware 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:
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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 then returns to normal operation. The NSF-aware router will look for alternate paths (go active) for any routes that are not refreshed by the NSF-capable (restarting router). The NSF-aware router will then return to normal operation. If all paths are refreshed by the NSF-capable router, the NSF-aware router will immediately return to normal operation.
Note
OSPF Operation
When an OSPF NSF-capable router performs an RP switchover, it must perform two tasks in order to resynchronize its Link State Database with its OSPF neighbors. First, it must relearn the available OSPF 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.
As quickly as possible after an RP switchover, the NSF-capable router sends an OSPF NSF signal to neighboring NSF-aware devices. Neighbor networking devices recognize this signal as a cue that the neighbor relationship with this router should not be reset. As the NSF-capable router receives signals from other routers on the network, it can begin to rebuild its neighbor list.
Once neighbor relationships are reestablished, the NSF-capable router begins to resynchronize its database with all of its NSF-aware neighbors. At this point, the routing information is exchanged between the OSPF neighbors. Once this exchange is complete, 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.
Note
The OSPF RFC 3623 graceful restart feature allows you to configure IETF NSF in multivendor networks. For more information, see OSPF RFC 3623 Graceful Restart, Cisco IOS Release 12.2(31)SB2.
IS-IS Operation
When an IS-IS NSF-capable router 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 routers on a network segment are NSF-aware, meaning that neighbor routers are running a software version that supports the IETF Internet draft for router restartability, they will assist an IETF NSF router which is restarting. With IETF, neighbor routers 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.
Note
If the neighbor routers 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
Using the IETF IS-IS configuration, as quickly as possible after an RP switchover, the NSF-capable router sends IS-IS NSF restart requests to neighboring NSF-aware devices. Neighbor networking devices recognize this restart request as a cue that the neighbor relationship with this router should not be reset, but that they should initiate database resynchronization with the restarting router. As the restarting router receives restart request responses from routers 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
Using the Cisco configuration option, full adjacency and 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.
Note
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.
Cisco NSF Benefits
The Cisco NSF feature has several benefits, including the following:
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How to Implement Cisco NSF
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Configuring and Verifying CEF NSF
The CEF NSF feature operates by default while the networking device is running in SSO mode. No configuration is necessary.
The following task explains how to verify that CEF is NSF-capable.
SUMMARY STEPS
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DETAILED STEPS
Configuring and Verifying BGP NSF
The following task explains how to configure BGP for NSF. Repeat this task on each of the BGP NSF peer devices.
SUMMARY STEPS
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DETAILED STEPS
To verify NSF for BGP, you must check that the graceful restart function is configured on the SSO-enabled networking device and on the neighbor devices. The following task explains how to perform this function.
SUMMARY STEPS
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paths [reg-exp] | received prefix-filter | received-routes | routes | policy [detail]]]DETAILED STEPS
Configuring and Verifying EIGRP NSF
EIGRP NSF support is enabled by default. Distributed platforms that run a supporting version of Cisco IOS software can support full NSF capabilities. These routers can perform a restart operation and can support other NSF capable peers. Single processor platforms that run a supporting version of Cisco IOS software support only NSF awareness. These routers maintain adjacency and hold known routes for the NSF-capable neighbor until it signals that it is ready for the NSF-aware router to send its topology table or the route-hold timer expires.
Note
The following task explains how to configure EIGRP for NSF. Repeat this procedure on each of the EIGRP NSF peer devices.
SUMMARY STEPS
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DETAILED STEPS
To verify NSF for EIGRP, you must check that NSF awareness and/or capability is enabled on the SSO-enabled networking device and on the neighbor devices. The following task explains how to perform this function.
SUMMARY STEPS
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DETAILED STEPS
Configuring and Verifying OSPF NSF
Note
The following task explains how to configure OSPF for NSF.
SUMMARY STEPS
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DETAILED STEPS
The following task explains how to verify OSPF for NSF.
SUMMARY STEPS
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DETAILED STEPS
Configuring and Verifying IS-IS NSF
The following task describe how to configure NSF for IS-IS.
SUMMARY STEPS
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DETAILED STEPS
To verify NSF for IS-IS, you must check that the NSF function is configured on the SSO-enabled networking device. The following task describes how to verify NSF for IS-IS.
SUMMARY STEPS
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DETAILED STEPS
Troubleshooting Tips
To troubleshoot the NSF feature, use the following commands in privileged EXEC mode, as needed:
The following tips may help you troubleshoot the device.
The system displays FIB errors.
Use the show cef state command to verify that distributed CEF switching is enabled on your platform. To enable distributed CEF, enter the ip cef distributed command in global configuration mode on the active RP.
Note
Cannot determine if an OSPF neighbor is NSF-aware.
To verify whether an OSPF neighbor device is NSF aware and if NSF is operating between them, use the show ip ospf neighbor detail command.
