IP Addresses and Services Configuration Guide for Cisco 8000 Series Routers, IOS XR Releases

Nonstop routing for transport sessions

Updated: September 8, 2026

Want to summarize with AI?

Explains how nonstop routing maintains supported routing protocol sessions during route processor failover by synchronizing application and transport state, and describes supported clients, failure handling, restrictions, and recovery behavior.

Nonstop routing (NSR) is a high-availability capability that

keeps supported routing protocol sessions active during route processor failover
synchronizes application and transport state between active and standby route processors, and
reduces session loss and packet loss during supported failures.

NSR supports Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and Label Distribution Protocol (LDP) during route processor failover. It also supports an OSPF, LDP, or TCP process crash and online insertion and removal.

Table 1. Feature History Table
Feature Name	Release	Description
Nonstop routing	Release 26.1.1	Introduced in this release on: Centralized Systems (8400 [ASIC: K100])(select variants only) This feature is supported on Cisco 8404-SYS-D router with 8404-RSP1-48-EM route processor.
Nonstop routing	Release 24.4.1	Introduced in this release on: Centralized Systems (8600 [ASIC: Q200], (select variants only); Modular Systems (8800 [LC ASIC: P100])(select variants only) NSR ensures uninterrupted routing by maintaining protocol sessions during process crashes, thus enhancing network reliability. It eliminates session loss and minimizes packet drop by preserving the router's state and synchronizing it with standby processes. This reliability is particularly beneficial for maintaining stable connections in dynamic environments. By automatically synchronizing routing information, NSR provides seamless network operation and reduces the need for manual intervention during failures. This feature is essential for environments demanding continuous network availability and high resilience to faults. Previously this feature was supported on Q200 and Q100. This feature is now supported on: Cisco 8608 router with 8608-RP route processor Cisco 8800 series routers with 8800-RP or 8800-RP2 router processors

Related routing information

For OSPF-specific NSR information, see the chapter about implementing OSPF in the Routing Configuration Guide for Cisco 8000 Series Routers.

How NSR preserves sessions during failures

Nonstop routing (NSR) keeps supported routing sessions and transport state synchronized between the active and standby route processors.

Summary

The session-preservation process uses these components:

Active route processor: Runs the active routing applications and TCP sessions.
Standby route processor: Maintains synchronized application and transport state.
NSR-enabled clients: Use the synchronized state to preserve their protocol sessions.

Workflow

These stages describe how NSR preserves sessions during failures:

During normal operation, NSR mirrors the application and TCP state from the active route processor to the standby route processor.
When a route processor fails over, the synchronized TCP sessions migrate to the standby route processor without notifying their peers.
When an active NSR-enabled process crashes and process-failure switchover is configured, the router can use route processor failover as the recovery action.
A fault-management policy can also handle an active process failure. The failing process does not generate the logs required to determine the root cause when this policy handles the failure.
When a standby NSR-enabled process restarts or crashes, active sessions remain established, but NSR protection is unavailable until the standby instances restart and resynchronize the sessions.

Result

Supported protocol sessions remain established across route processor failover. Without NSR, protocols on the new active route processor must reestablish their sessions. Graceful restart can reduce traffic loss, but it does not provide the same session continuity.

NSR restrictions and supported clients

Review the restrictions and client support before you configure process-failure switchover for nonstop routing (NSR).

Process-failure switchover has these restrictions:

The router does not trigger route processor failover when a process restarts, even when nsr process-failures switchover is configured.
The router triggers route processor failover only when a process crashes.

The process-failure switchover feature supports these NSR clients:

Layer 2 VPN (L2VPN)
Virtual Private Dialup Network (VPDN)
TCP
Intermediate System-to-Intermediate System (IS-IS)
Open Shortest Path First (OSPF)
Multiprotocol Label Switching Label Distribution Protocol (MPLS LDP)
Border Gateway Protocol (BGP)

Configure failover as a recovery action

Use this procedure to maintain nonstop routing (NSR) by switching active instances to their standby instances after a supported process crash.

When active TCP or an NSR client of active TCP crashes, its TCP sessions go down. With process-failure switchover configured, the router initiates route processor switchover when active TCP or an active application, such as Label Distribution Protocol (LDP) or Open Shortest Path First (OSPF), crashes.

For information about configuring Multiprotocol Label Switching LDP for NSR, see the MPLS Configuration Guide for Cisco 8000 Series Routers. For per-process NSR configuration, see the Routing Configuration Guide for Cisco 8000 Series Routers.

Before you begin

You must belong to a user group associated with a task group that includes the required task IDs. If a user group assignment prevents command access, contact your authentication, authorization, and accounting administrator.

Procedure

Configure process-failure switchover.

Example:

Router# configure
Router(config)# nsr process-failures switchover
Router(config)# commit

Verify the process-failure switchover configuration.

Example:

Router# show running-configuration nsr process-failures switchover
nsr process-failures switchover

The router uses route processor failover as the recovery action when a supported active NSR-enabled process crashes.

Need help?

Open a support case

(Requires a Cisco Service Contract)

The documentation set for this product strives to use bias-free language. For the purposes of this documentation set, bias-free is defined as language that does not imply discrimination based on age, disability, gender, racial identity, ethnic identity, sexual orientation, socioeconomic status, and intersectionality. Exceptions may be present in the documentation due to language that is hardcoded in the user interfaces of the product software, language used based on RFP documentation, or language that is used by a referenced third-party product. Learn more about how Cisco is using Inclusive Language.