IS-IS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

PDF

IS-IS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

IS-IS non-stop forwarding

Updated: June 9, 2026

Want to summarize with AI?

Log in

Introduces IS-IS nonstop forwarding (NSF) support and provides instructions for configuring nonstop forwarding to ensure uninterrupted protocol operations during control plane restart.

IS-IS non-stop forwarding (NSF) is a network resiliency function that

  • minimizes the amount of time a network is unavailable during IS-IS process restarts

  • suppresses routing flaps, maintaining routing stability and performance, and

  • allows data forwarding to continue while routing protocol state is restored.

IETF NSF: An NSF implementation based on RFC 5306 standards, requiring neighbor routers with NSF-aware software to assist in restoring routing information after a restart.

Cisco NSF: A proprietary implementation that stores the state needed for restart and recovers it using standard IS-IS protocol features, so it works even if neighboring devices do not support RFC 5306.

Impact of restarts on IS-IS adjacencies

When a router running IS-IS restarts, it typically loses adjacency with its peers, causing routing flaps that destabilize the network. NSF suppresses these flaps, keeps routes stable, and ensures minimal disruption. During an IS-IS restart, the router must relearn its IS-IS neighbors without resetting relationships and reacquire the network link-state database.

NSF deployment options

IS-IS NSF can be configured using two options:

  • IETF NSF: Relies on cooperation from NSF-aware neighbor routers to rebuild routing information.

  • Cisco NSF: Functions independently using standard IS-IS features and does not need neighbor support.

Behavior when neighbor routers lack IETF NSF support

If IETF NSF is enabled and a neighboring router does not support it, adjacency may flap, but nonstop forwarding continues for compatible neighbors. If no neighbors support IETF NSF, a restart falls back to a full cold start.

Hello intervals and BFD considerations for NSF

IS-IS protocol supports shorter hello intervals for neighbor management, but nonstop forwarding works only with the default hello interval. Fast connectivity loss detection is handled by Bidirectional Forwarding Detection (BFD), so hello intervals do not need adjustment for nonstop forwarding.

Examples

When configuring IS-IS NSF, if neighbors support RFC 5306, they will help restore routing information after a restart. Otherwise, Cisco NSF will work without neighbor support, maintaining traffic forwarding using protocol-defined mechanisms.

In a mixed-vendor environment, Cisco NSF enables resilience without relying on the IETF standard implementation.

Configure nonstop forwarding for IS-IS

Ensure IS-IS continues routing traffic and synchronizing link-state information with neighbors across process restarts, such as RP failovers or IS-IS software upgrades, minimizing disruptions and user session loss.

Nonstop forwarding (NSF) enables the router to quickly recover from certain process restarts without causing IS-IS routing interruptions. This is important for service continuity in environments where link flaps or session loss is unacceptable.

Procedure

1.

Enable IS-IS routing for the required routing instance.

Example:

Router# configure
Router(config)# router isis isp
2.

Enable NSF and select the protocol based on your network environment.

  • For heterogeneous networks

    Router(config-isis)# nsf cisco

  • For homogeneous (IETF draft-based) networks

    Router(config-isis)# nsf ietf
Note

Use cisco if not all adjacent devices support NSF. Use ietf if all adjacent devices support IETF draft-based restartability.
3.

Configure the number of permitted NSF restart acknowledgment resends.

Example:

Router(config-isis)# nsf interface-expires 1

If the resend limit is reached, NSF falls back to a cold restart.
4.

Configure the number of seconds to wait for each restart acknowledgment.

Example:

Router(config-isis)# nsf interface-timer 15
5.

Configure the maximum route lifetime following an NSF restart.

Example:

Router(config-isis)# nsf lifetime 20
Note

Set this value to match the time required for a full NSF restart. If set too high, stale routes may persist; if too low, valid routes may be purged too soon.
6.

Use the show running-config router isis isp command to check if NSF settings appear under the relevant IS-IS instance.