What is Cisco Non Stop Forwarding (NSF) and How Does It Work?

Network uptime is crucial for any organization. Non Stop Forwarding (NSF) is a vital Cisco IOS Software feature designed to maximize network service availability and protect against unexpected downtime from hardware or software issues. Its primary goal is to continue forwarding IP packets even when a Route Processor (RP) experiences a problem or undergoes a switchover. This capability dramatically reduces network instability by preventing widespread routing flaps that typically occur when a network device restarts.

What is Cisco Non-Stop Forwarding?

Cisco NSF is a feature that ensures continuous IP packet forwarding during RP failovers. What is Cisco non-stop forwarding? It separates the control plane from the data plane, allowing the data plane—powered by Cisco Express Forwarding (CEF)—to continue forwarding packets using the last known forwarding information base (FIB) even when the control plane restarts. This mechanism minimizes downtime and keeps traffic flowing smoothly during failovers.

How Cisco NSF Works

Cisco NSF functions by separating the control plane from the data plane. The data plane continues forwarding traffic using cached FIB information during an RP switchover. Meanwhile, the control plane restarts on the newly active RP. NSF-capable routers notify their NSF-aware neighbors of the restart. These neighbors continue forwarding traffic to the restarting router and assist in rebuilding its routing information base (RIB). Once routing protocols reconverge, CEF updates the FIB and removes stale routes.

What is the Difference Between NSF and SSO?

It’s common to ask, what is the difference between NSF and SSO? Stateful Switchover (SSO) is the underlying high-availability infrastructure that maintains Layer 2 connectivity and mirrors platform and protocol state between redundant Route Processors, preventing link flaps during a switchover. NSF builds on SSO by ensuring Layer 3 packet forwarding continues uninterrupted during the RP switchover by leveraging the existing forwarding tables while routing protocols re-establish their routing databases.

What is the Difference Between NSF and NSR?

Another common question is, what is the difference between NSF and NSR? NSR, or Non Stop Routing, is an extension of NSF that not only maintains forwarding but also preserves routing protocol sessions and state during failover. NSR depends on NSF and is usually supported on advanced Cisco platforms to provide even higher network availability by ensuring routing protocol continuity in addition to forwarding.

Cisco Platforms Supporting NSF

NSF with SSO is supported on many Cisco devices, including:

• Cisco 10000, 12000 Series Internet Routers
• Cisco Catalyst 6500, 4500, 9400, and 9500 Series Switches
• Cisco 7600, 7500 Series Routers
• Cisco ASR 1000 Series Aggregation Services Routers

Note that some single-processor platforms are NSF-aware but not NSF-capable due to hardware limitations.

How to Configure Cisco NSF

To enable NSF, first configure SSO, then activate NSF for supported routing protocols.

Device(config)# redundancy
Device(config-red)# mode sso
Device(config-red)# end

Enable BGP Graceful Restart:

Router(config)# router bgp 
Router(config-router)# bgp graceful-restart

Enable OSPF NSF:

Router(config)# router ospf 
Router(config-router)# nsf

Enable EIGRP NSF:

Router(config)# router eigrp 
Router(config-router)# nsf

Enable IS-IS NSF:

Router(config)# router isis 
Router(config-router)# nsf [cisco | ietf]

How to Verify Cisco NSF Status

• show redundancy states
• show ip bgp neighbors (check for Graceful Restart Capability)
• show ip ospf nsf
• show isis nsf
• show ip protocols (for EIGRP NSF)

Use Cases and Benefits of Cisco NSF

NSF is highly beneficial in environments requiring high availability such as service provider edges, enterprise networks, data centers, and campus networks. The key benefits include:

• Improved network uptime by maintaining continuous forwarding
• Reduced routing instability and flaps
• Seamless switchover transparent to neighbors
• Preservation of user sessions and ongoing flows

Troubleshooting and Best Practices

• Always enable SSO before NSF
• Ensure neighbors are NSF-aware to prevent asymmetric routing
• Explicitly configure BGP graceful restart
• Tune protocol timers for optimal convergence
• Verify hardware and software compatibility
• Use show and debug commands for troubleshooting

Frequently Asked Questions about Non Stop Forwarding (NSF)

Q1: What is Cisco Non-Stop Forwarding?

Cisco NSF ensures continuous IP packet forwarding during Route Processor failovers, minimizing network downtime and maintaining traffic flow.

Q2: What is the difference between NSF and SSO?

SSO maintains platform and Layer 2 state between redundant processors, while NSF ensures Layer 3 forwarding continuity during RP switchover.

Q3: What is the difference between NSF and NSR?

NSR extends NSF by preserving routing protocol sessions and state, requiring NSF to function and providing enhanced high availability.

Q4: How to stop forwarding calls on Cisco?

This question relates to Cisco call forwarding in telephony, which is unrelated to network packet forwarding and Cisco NSF. For call forwarding configuration, refer to Cisco Unified Communications documentation.

Q5: How do I find port number on a Cisco switch?

Use show interface status to list ports with their logical IDs and physical status for easy identification.

Q6: What is the Cisco port naming convention?

Cisco port names follow the format TypeX/Y/Z, where X is the stack member, Y is the module number (0 for built-in), and Z is the port number.

Q7: What is the console port on Cisco 9200L switch?

The Cisco 9200L has both USB Type-B and RJ-45 console ports for CLI access and device management.