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

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

Configure SPF interval for an IS-IS single-topology address family

Updated: June 9, 2026

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Provides instructions for configuring the SPF interval within a single-topology IS-IS environment to control the timing of shortest path calculations.

Adjust SPF calculation timing to tune router performance in IS-IS single-topology configurations.

IS-IS uses Shortest Path First (SPF) calculations to compute routes for each topology. In single-topology mode, only IPv4 requires SPF; IPv6 borrows the IPv4 topology and does not require a separate SPF calculation. Tuning SPF intervals can improve router performance and prevent resource issues. Incremental SPF (ISPF) can be enabled separately, allowing more efficient recalculation after topology changes. The SPF interval determines the minimum time between SPF runs.

Before you begin

  • Ensure that IS-IS routing is enabled on your device.

  • Understand the impact of SPF interval values on network stability and CPU utilization.

Follow these steps to configure the SPF interval for an IS-IS single-topology address family:

Procedure

1.

Enter IS-IS routing configuration mode for the specified instance, and define the address family for single-topology mode:

Example:

Router# configure
Router(config)# router isis isp
Router(config-isis)# address-family ipv4 unicast

You can change the level of routing to be performed by a particular routing instance by using the is-type router configuration command.
2.

Set the desired SPF interval values as needed.

Example:


Router(config-isis-af)# spf-interval initial-wait 10 maximum-wait 30

  • These values control the minimum time between consecutive SPF calculations.

  • If set too low, excessive SPF runs may waste CPU resources in unstable networks.

  • If set too high, topology change reactions may be delayed, increasing the risk of lost packets.

  • SPF interval does not apply to incremental SPF (ISPF).
3.

(Optional) Enable incremental SPF calculation.

Example:


Router(config-isis-af)# ispf

This enables ISPF, which recalculates only portions of the topology affected by a change.
4.

(Optional) Verify the SPF settings and calculation frequency.

Example:


Router# show isis instance 1 spf-log ipv4

Displays how often and why the router has run a full SPF calculation.