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

IS-IS SPF interval delay

Updated: June 9, 2026

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Explains IS-IS SPF interval delay concepts, outlining the effects of SPF delay on IS-IS operations and supplying procedures for configuring IETF-compliant SPF calculation intervals to manage route computation frequency.

IS-IS SPF interval delay is a protocol timing mechanism that

  • defines a standard algorithm to postpone SPF computations in IS-IS

  • reduces computational load and churn on IGP nodes when multiple, temporally close network events trigger several SPF calculations, and

  • lowers the probability and the duration of transient forwarding loops during native IS-IS convergence.

Table 1. Feature History Table

Feature Name

Release

Description

Setting SPF interval in IS-IS to postpone the IS-IS SPF computations

Release 25.1.1

Introduced in this release on: Fixed Systems (8700 [ASIC: K100], 8010 [ASIC: A100])(select variants only*)

*This feature is supported on:

  • 8712-MOD-M

  • 8011-4G24Y4H-I

Setting SPF interval in IS-IS to postpone the IS-IS SPF computations

Release 24.4.1

Introduced in this release on: Fixed Systems (8200 [ASIC: P100], 8700 [ASIC: P100])(select variants only*); Modular Systems (8800 [LC ASIC: P100])(select variants only*)

*This feature is supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 88-LC1-36EH

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

Setting SPF interval in IS-IS to postpone the IS-IS SPF computations

Release 7.7.1

You can now define a standard algorithm to postpone the IS-IS SPF computations by setting an SPF interval. This reduces the computational load and churn on IGP nodes when multiple temporally close network events trigger multiple SPF computations.

This algorithm also reduces the probability and the duration of transient forwarding loops during native IS-IS convergence when the protocol reacts to multiple temporally close events.

This feature complies with RFC 8405 .

This feature introduces the spf-interval ietf command.

Additional details

The algorithm specified in RFC 8405 provides a standardized approach to temporarily postpone IS-IS SPF computation. Applying this feature is optional but can be beneficial in environments with high protocol churn.

Configure IETF-compliant SPF calculation intervals

Postponing SPF calculations by configuring intervals helps prevent excessive CPU utilization and stabilizes routing updates in environments experiencing frequent network changes. The IETF algorithm is recommended for most deployments, as per RFC 8405.

Procedure

1.

Enable IS-IS routing for the target instance and enter router configuration mode.

Example:

Router(config)# router isis isp
2.

Specify the address family (IPv4 or IPv6) for unicast routing and enter address family configuration mode.

Example:

Router(config-isis)# address-family ipv4 unicast
3.

Set the SPF calculation interval type to IETF.

Example:

Router(config-isis-af)# spf-interval ietf
4.

Adjust delay values if needed for your environment using parameters like short-wait, initial-wait, and so on.

Example:

Router(config-isis-af)# spf-interval ietf short-wait 500
5.

Verify the new SPF interval configuration.

Example:

Router# show run router isis
router isis 1
 net 49.0001.0000.0000.0100.00
 log adjacency changes
 address-family ipv4 unicast
  metric-style wide
  spf-interval ietf
 !
 address-family ipv6 unicast
  metric-style wide
  spf-interval ietf
 !
6.

Monitor SPF calculation logs for expected delays and triggers.

Example:

Router# show isis ipv4 spf-log last 5 detail
IS-IS 1 Level 2 IPv4 Unicast Route Calculation Log
                    Time Total Trig.
Timestamp    Type   (ms) Nodes Count First Trigger LSP    Triggers
------------ ----- ----- ----- ----- -------------------- -----------------------
--- Wed Mar 16 2022 ---
15:31:49.763  FSPF     1     6     3         tb5-r4.00-00 LINKBAD PREFIXBAD
  Delay:                 101ms (since first trigger)                  
                         261177ms (since end of last calculation)
  Trigger Link:          tb5-r2.00
  Trigger Prefix:        34.1.24.0/24
  New LSP Arrivals:      0
  SR uloop:              No
  Next Wait Interval:    200ms					    
  RIB Batches:           1 (0 critical, 0 high, 0 medium, 1 low)
  Timings (ms):          +--Total--+
                          Real   CPU
    SPT Calculation:         1     1
    Route Update:            0     0
                         ----- -----

It is recommended to use the default delay values, which are listed in Syntax description . These default parameters are suggetsed by RFC 8405 . These should be appropriate for most networks.

However, you can configure different values if required.