Cisco Configuring Nonstop Forwarding with Stateful Switchover User Guide PDF

Prerequisites for Cisco Nonstop Forwarding with Stateful Switchover

Configuring Nonstop Forwarding with Stateful Switchover Cisco nonstop forwarding (NSF) works with the stateful switchover (SSO) feature. NSF works with SSO to minimize the amount of time a network is unavailable to users following a switchover. The main objective of NSF SSO is to continue forwarding IP packets following a Route Processor (RP) switchover.

Prerequisites for Cisco Nonstop Forwarding with Stateful Switchover, on page 1 Restrictions for Cisco Nonstop Forwarding with Stateful Switchover, on page 2 Information About Cisco Nonstop Forwarding with Stateful Switchover, on page 2 How to Configure Cisco Nonstop Forwarding with Stateful Switchover, on page 7 Verifying Cisco Express Forwarding with Cisco Nonstop Forwarding, on page 8 Configuration Examples for Cisco Nonstop Forwarding with Stateful Switchover, on page 9 Additional References for Cisco Nonstop Forwarding with Stateful Switchover, on page 9 Feature History for Cisco Nonstop Forwarding with Stateful Switchover, on page 9 Prerequisites for Cisco Nonstop Forwarding with Stateful Switchover • Cisco NSF must be configured on a networking device that has been configured for SSO.

• Border Gateway Protocol (BGP) support in NSF requires that neighbor networking devices be NSF-aware; that is, 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 does not establish an NSF-capable session with that neighbor. All other neighbors that have graceful restart capability continue to have NSF-capable sessions with this NSF-capable networking device. • Open Shortest Path First (OSPF) support in NSF requires that all neighbor networking devices be NSF-aware.

If an NSF-capable device discovers that it has non-NSF-aware neighbors on a particular network segment, it disables NSF capabilities for that segment. Other network segments composed entirely of NSF-capable or NSF-aware devices continue to provide NSF capabilities. Configuring Nonstop Forwarding with Stateful Switchover 1

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Overview of Cisco Nonstop Forwarding with Stateful Switchover

Restrictions for Cisco Nonstop Forwarding with Stateful Switchover The following are restrictions for configuring NSF with SSO: • For NSF operation, you must have SSO configured on the device. • All Layer 3 neighboring devices must be an NSF helper or NSF-capable to support graceful restart capability. • For IETF, all neighboring devices must be running an NSF-aware software image. • The Hot Standby Routing Protocol (HSRP) is not supported with NSF SSO. • An NSF-aware device cannot support two NSF-capable peers performing an NSF restart operation at the same time. However, both neighbors can reestablish peering sessions after the NSF restart operation is complete.

Information About Cisco Nonstop Forwarding with Stateful Switchover Overview of Cisco Nonstop Forwarding with Stateful Switchover Cisco NSF works with the SSO feature. The device supports fault resistance by allowing a standby switch to take over if the active device becomes unavailable. NSF works with SSO to minimize the amount of time a network is unavailable. 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 with SSO 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/SSO, peer networking devices do not experience routing flaps. Data traffic is forwarded through intelligent line cards or dual forwarding processors (FPs) while the standby router processor (RP) assumes control from the failed active RP during a switchover.

NSF with SSO operation provides the ability of line cards and FPs to remain active through a switchover and to be kept current with the Forwarding Information Base (FIB) on the active RP. NSF provides the following benefits: • Improved network availability—NSF continues forwarding network traffic and application state information so that user session information is maintained after a switchover. • Overall network stability—Network stability can be improved with the reduction in the number of route flaps that are created when devices in the network fail, and lose their routing tables.

Configuring Nonstop Forwarding with Stateful Switchover Restrictions for Cisco Nonstop Forwarding with Stateful Switchover Configuring Nonstop Forwarding with Stateful Switchover 2

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Cisco Nonstop Forwarding Operation

• Neighboring devices do not detect a link flap—Because interfaces remain active during a switchover, neighboring devices do not detect a link flap (the link does not go down and come back up). • Prevents routing flaps—Because SSO continues forwarding network traffic during a switchover, routing flaps are avoided. • Maintains user sessions established prior to the switchover. SSO Operation When a standby device runs in SSO mode, the standby device starts up in a fully-initialized state and synchronizes with the persistent configuration and the running configuration on the active device.

It subsequently maintains the state of the protocols, and all changes in hardware and software states for features that support SSO are kept in synchronization. Consequently, it offers minimum interruption to Layer 2 sessions in a redundant active device configuration. If the active device fails, the standby device becomes the active device. This new active device uses existing Layer 2 switching information to continue forwarding traffic. Layer 3 forwarding is delayed until routing tables are repopulated in the newly active device. The routing tables require around 80 seconds for repopulation.

You can use the show ip bgp ip-address command, in privileged EXEC mode, to check whether the routing tables are repopulated or not. Note Cisco Nonstop Forwarding Operation NSF always runs with SSO, and provides redundancy for Layer 3 traffic. NSF is supported by BGP, Enhanced Interior Gateway Routing Protocol (EIGRP), and OSPF routing protocols and also by Cisco Express Forwarding for forwarding. These routing protocols have been enhanced with NSF-capability and awareness, which means that devices running these protocols can detect a switchover and take necessary actions to continue forwarding network traffic and to recover route information from peer devices.

Each protocol depends on Cisco Express Forwarding to continue forwarding packets during switchover while routing protocols rebuild the Routing Information Base (RIB) tables. After the convergence of routing protocols, Cisco Express Forwarding updates the FIB table and removes stale route entries. Cisco Express Forwarding then updates the hardware with the new FIB information. If the active device is configured (with the graceful-restart command) for BGP, OSPF, or EIGRP routing protocols, routing updates are automatically sent during the active device election. NSF has two primary components: • NSF-aware: A networking device is NSF-aware if it is running NSF-compatible software.

If neighboring devices detect that an NSF device can still forward packets when an active device election happens, this capability is referred to as NSF-awareness. Enhancements to the Layer 3 routing protocols (BGP, OSPF, and EIGRP) are designed to prevent route-flapping so that the Cisco Express Forwarding routing table does not time out or the NSF device does not drop routes. An NSF-aware device helps to send routing protocol information to the neighboring NSF device. NSF-awareness is enabled by default for EIGRP-stub, EIGRP, and OSPF protocols. NSF-awareness is disabled by default for BGP. • NSF-capability:

A device is NSF-capable if it is configured to support NSF; it rebuilds routing information from NSF-aware or NSF-capable neighbors. NSF works with SSO to minimize the amount of time that Configuring Nonstop Forwarding with Stateful Switchover SSO Operation Configuring Nonstop Forwarding with Stateful Switchover 3

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BGP Operation

a Layer 3 network is unavailable following an active device election by continuing to forward IP packets. Reconvergence of Layer 3 routing protocols (BGP, OSPFv2, and EIGRP) is transparent to the user and happens automatically in the background. Routing protocols recover routing information from neighbor devices and rebuild the Cisco Express Forwarding table. Cisco Express Forwarding A key element of NSF is packet forwarding. In a Cisco networking device, packet forwarding is provided by Cisco Express Forwarding.

Cisco Express Forwarding maintains the Forwarding Information Base (FIB), and uses the FIB information that is current at the time of a switchover to continue forwarding packets during a switchover, to reduce traffic interruption during the switchover. During normal NSF operation, Cisco Express Forwarding on the active device synchronizes its current FIB and adjacency databases with the FIB and adjacency databases on the standby device . Upon switchover, the standby device initially has FIB and adjacency databases that are mirror images of those that were current on the active device .

Cisco Express Forwarding keeps the forwarding engine on the standby device current with changes that are sent to it by Cisco Express Forwarding on the active device . The forwarding engine can 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 cause prefix-by-prefix updates to Cisco Express Forwarding, which it uses to update the FIB and adjacency databases. Existing and new entries receive the new version (“epoch”) number, indicating that they have been refreshed. The forwarding information is updated on the forwarding engine during convergence.

The device 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 Routing protocols run only on the active RP, and receive routing updates from neighbor devices. Routing protocols do not run on the standby RP. Following a switchover, routing protocols request that the NSF-aware neighbor devices send state information to help rebuild routing tables.

Alternately, the Intermediate System-to-Intermediate System (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, routing protocols depend on Cisco Express Forwarding to continue forwarding packets while routing protocols rebuild the routing information. Note BGP Operation 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 this capability, it is aware that the device sending the message is NSF-capable. Both the NSF-capable device and its BGP peer(s) need to exchange the Graceful Restart Capability in their OPEN messages, at the time of session establishment. If both peers do not exchange the Graceful Restart Capability, the session is not graceful restart capable.

Configuring Nonstop Forwarding with Stateful Switchover Cisco Express Forwarding Configuring Nonstop Forwarding with Stateful Switchover 4

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