IS-IS OOR protection mechanisms

Updated: June 9, 2026

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Describes IS-IS protection enhancements specifically designed for out-of-resource (OOR) conditions, emphasizing mechanisms that strengthen protocol resiliency when resources are limited.

IS-IS protection mechanism is a network feature set that

continuously tracks IS-IS process memory usage to detect and respond to out-of-resource (OOR) conditions
applies corrective actions, such as process restarts and LSP flow control, to mitigate memory leaks and excessive route installation, and
ensures ongoing packet forwarding by using Nonstop Forwarding (NSF) during recovery actions to minimize network disruption.

OOR Condition: An OOR condition occurs when the IS-IS process uses more memory than its configured resource limit, potentially triggering process restarts or other protections.

Memory Leak: A memory leak describes a situation where the IS-IS process does not free memory that is no longer needed, gradually causing resource exhaustion.

Nonstop Forwarding (NSF): NSF is a mechanism allowing routers to keep forwarding packets without interruption, even while the IS-IS process restarts.

Table 1. Feature History Table
Feature Name	Release Name	Description
Protect IS-IS Processes in OOR Conditions	Release 25.4.1	Introduced in this release on: Fixed Systems (8700 [ASIC: K100])(select variants only) This feature is supported on Cisco 8711-48Z-M routers.
Protect IS-IS Processes in OOR Conditions	Release 25.1.1	Introduced in this release on: Fixed Systems (8010 [ASIC: A100])(select variants only) This feature is supported on Cisco 8011-4G24Y4H-I routers.
Protect IS-IS Processes in OOR Conditions	Release 24.4.1	Introduced in this release on: Fixed Systems (8200 [ASIC: P100], 8700 [ASIC: P100, K100])(select variants only); Modular Systems (8800 [LC ASIC: P100])(select variants only) This feature is supported on: 8212-48FH-M 8711-32FH-M 8712-MOD-M 88-LC1-36EH 88-LC1-12TH24FH-E 88-LC1-52Y8H-EM
Protect IS-IS Processes in OOR Conditions	Release 24.2.11	This feature enables prompt alerts for out-of-resource conditions in IS-IS processes that could otherwise cause network instability and disruption due to memory leaks and excessive link-state packets (LSPs). Additionally, you can disable the overload bit status flag in the router's LSP to prevent setting the overload-bit. We recommend consulting with Cisco for optimal results before making this change. Previously, during OOR conditions, IS-IS processes restarted themselves, but the OOR conditions could persist. This ability to protect IS-IS processes in OOR conditions is enabled by default and you can't disable it. The feature introduces these changes: CLI: The feature introduces fields that indicate the memory state of the IS-IS protocol in the show isis protocol command. oor-set-overload-bit disable command. YANG Data Model New XPaths for `Cisco-IOS-XR-clns-isis-cfg` `Cisco-IOS-XR-um-router-isis-cfg` (refer to GitHub and YANG Data Models Navigator)

Managing excessive link-state packets (LSPs)

Errors such as redistributing large numbers of prefixes from other protocols can result in excessive LSPs being sent to IS-IS. This flood of information can overwhelm router memory and CPU, potentially causing OOR conditions. IS-IS OOR protection mechanisms monitor and control LSP rates and route acceptance to maintain system stability.

Automatic resource limit monitoring

IS-IS regularly checks its memory usage against the maximum permitted value (RLIMIT_DATA). If memory consumption nears the threshold, alerts are generated and protective measures are triggered.

Suppression of FRR backup path computation

When IS-IS is not in the “Normal” memory state, Fast Reroute (FRR) backup path calculations and installations are temporarily disabled. When memory usage returns to “Normal” and remains so for at least 180 seconds, standard FRR operations resume.

When a memory leak is detected, the IS-IS process restarts to reclaim memory, with NSF ensuring that traffic continues to flow during recovery.
If IS-IS receives a sudden surge of routes, OOR mechanisms protect the router by regulating how many new routes and LSPs are processed.

Table 2. Memory usage states and associated monitoring intervals
Memory Usage (%)	Memory State	Periodic Monitoring Interval in Seconds
Less than 80	Normal	60
Greater than or equal to 80 and less than 90	Minor	30
Greater than or equal to 90 and less than 95	Severe	20
Greater than or equal to 95	Critical	10

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