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

Inter-area route propagation limits in IS-IS

Updated: September 8, 2026

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Describes how to control the number of prefixes that IS-IS propagates between levels and how to monitor propagated-prefix limits.

Inter-area route propagation limits in IS-IS are configuration parameters that

  • allow operators to specify the maximum number of route prefixes that can be propagated between IS-IS levels

  • provide an option to generate a warning when the configured limit is exceeded

  • when enforcement is enabled, stop advertising additional prefixes once the limit is reached

  • provide a warning-only mode in which all eligible prefixes are advertised, even when the limit is exceeded, and

  • help protect the network from excessive routing table growth caused by misconfiguration or unexpected routing protocol events.

Table 1. Feature History Table

Feature Name

Release Name

Description

Inter-area route propagation limits in IS-IS

Release 26.3.1

Introduced in this release on: Fixed Systems (8200 [ASIC: Q200, P100], 8700 [ASIC: P100, K100], 8010 [ASIC: A100]); Centralized Systems (8600 [ASIC: Q200]) ; Modular Systems (8800 [LC ASIC: Q200, P100])

This feature helps protect the network from excessive inter-area route propagation caused by misconfiguration or unexpected route growth by limiting the number of prefixes propagated between IS-IS levels.

This feature, using the new maximum-propagated-prefixes command, allows operators to define propagation thresholds, receive early warnings, and optionally stop advertising additional prefixes when limits are exceeded.

This feature introduces these changes:

CLI: A new command, maximum-propagated-prefixes, has been introduced.

Inter-area route propagation limits in IS-IS

In IS-IS, routes learned in one level may be propagated into another level to support inter-area reachability. Prior to this enhancement, operators could control which routes were propagated by using policy, but they could not directly limit the number of propagated prefixes.

Starting with Cisco IOS XR 26.3.1, this feature adds a new command, maximum-propagated-prefixes, which allows operators to define a propagation threshold for inter-area routes. When the configured threshold is approached or exceeded, the router can generate warnings and, depending on configuration, either continue propagating routes or stop advertising additional prefixes. This feature helps protect the network from unexpected route growth between IS-IS levels.

When the propagated-prefix count exceeds the configured threshold, the router can either:

  • generate a warning and continue advertising routes, or

  • stop advertising additional propagated routes.

Benefits of inter-area route propagation limits in IS-IS

  • Improved operational protection against accidental large-scale route propagation.

  • Better control of IS-IS inter-area advertisements.

  • Configurable warning thresholds for earlier visibility into route growth.

  • Operational consistency with the existing IS-IS redistributed-prefix limiting model.

Deployment models for inter-area route propagation limits in IS-IS

The inter-area route propagation limits in IS-IS feature supports two deployment models:

  • Warning-only mode (soft limit): The router generates a warning when the configured number of prefixes exceeds the soft limit, but continues advertising routes.

  • Enforced mode (hard limit): The router generates a warning when a certain percentage of the hard limit is reached. IS-IS never advertises more propagated prefixes than the configured hard limit. If the number of eligible prefixes exceeds the hard limit, excess prefixes are omitted from the local system’s LSPs.

These deployment models enable operators to monitor or strictly control the propagation of inter-area routes, helping prevent excessive route advertisement and maintain stable network operations. Use warning-only mode for monitoring purposes, and enforced mode when strict limits must be upheld.

Restrictions for inter-area route propagation limits in IS-IS

When you configure propagated-prefix limits in IS-IS, consider:

  • Use route policy to control which prefixes are eligible for propagation.

  • Use maximum-propagated-prefixes to control how many eligible prefixes IS-IS advertises.

  • This feature applies to inter-area propagated routes. It does not limit redistributed routes.

  • To limit redistributed routes, use maximum-redistributed-prefixes .

  • To control propagation from level 2 into level 1, first enable level 2 into level 1 in IS-IS.

  • Level 1 limits prefixes propagated into level 1.

  • Level 2 limits prefixes propagated into level 2.

  • Omit the level parameter so that the configuration applies to both levels.

  • Limits are evaluated independently for the applicable area and topology.

Defaults and ranges for inter-area route propagation limits in IS-IS

This reference details the configurable ranges and default behaviors for inter-area route propagation limits in IS-IS:

  • Prefix limit range: 1–100000

  • Warning-percentage range: 20–99

  • Default warning percentage in enforced mode: 70%

  • Default behavior without explicit configuration: warning-only at 20000 prefixes

Use these values to configure and monitor prefix propagation for IS-IS inter-area routes efficiently.

Configure a hard limit threshold on propagated routes

Uncontrolled IS-IS route propagation can result in excessive LSP growth and increased downstream routing state. By setting a hard limit and warning thresholds, you prevent the system from advertising more routes than intended, maintaining network stability and visibility.

Before you begin

  • Ensure IS-IS is enabled on the router.

  • Confirm the target propagation level is part of your IS-IS design.

  • Have any required route policy configured.

Follow these steps to configure a hard limit and warning-only threshold on propagated routes:

Procedure

  1. Enter IS-IS address-family configuration mode for the target instance.

    Example:

    Router# config
Router(config)# router isis core
Router(config-isis)# address-family ipv4 unicast
Router(config-isis-af)# propagate level 2 into level 1 route-policy pass-all
Router(config-isis-af)# maximum-propagated-prefixes 1000 warning-percentage 50

  2. Configure the maximum propagated prefix limit for the desired level in one of the following ways.

    • This example configures the hard limit for level 2.

      Router(config-isis-af)# maximum-propagated-prefixes 100 level 2

    • This example changes the default early-warning percentage from 70% to the specified value.

      Router(config-isis-af)# maximum-propagated-prefixes 100 warning-percentage 50 level 2

      The router tracks the number of propagated prefixes for the configured level. When the count exceeds the configured warning threshold, the router generates a warning. When the count exceeds the configured hard limit, the router stops advertising additional propagated routes.

      A warning message is displayed when the propagated prefix count exceeds the configured hard limit. 
      Router(config-isis-af)# maximum-propagated-prefixes 100 warning-percentage 50 level 2
RP/0/1/CPU0:Aug 24 12:52:31.457 CEST: isis[1009]: %ROUTING-ISIS-4-MAX_PROPAGATED_PREFIXES : ISIS (eu): Topology IPv4 Unicast is exceeding the maximum number of propagated prefixes (100) into level 2

    • This example configures the warning-only threshold that does not enforce a hard limit.

      Router(config-isis-af)# maximum-propagated-prefixes 90 warning-only level 2

      The router generates warnings when the propagated-prefix count exceeds the configured threshold, but it continues to advertise propagated routes.

      A warning message is displayed when the propagated prefix count exceeds the soft limit. 
      Router(config-isis-af)# maximum-propagated-prefixes 150 warning-percentage 50 level 2
RP/0/1/CPU0:Aug 24 12:50:03.227 CEST: isis[1009]: %ROUTING-ISIS-4-MAX_PROPAGATED_PREFIXES : ISIS (eu): Topology IPv4 Unicast is exceeding the soft limit of propagated prefixes (150) into level 2

  3. Run the show isis protocol to verify propagated-prefix limits and review the advertised-prefix section for the relevant level.

    Example:

    Router# show isis protocol
IS-IS Router:                      eu
  System Id:                       0000.0000.0005
  Hostname:                        ios
  IS Levels:                       level-1-2
  Manual area address:             49.0002
  Routing for area address:        49.0002
  Multi-Instance Id:               0
  Running on:                      RP/0/1/CPU0
  Job Id:                          1009
  PID:                             58446

  Started:                         Tue 2026 Sep 08 14:42:26.442 CEST
    Respawn count:                 3
    Null0 ready:                   Tue 2026 Sep 08 14:42:26.680 CEST
    First adjacency up:            Tue 2026 Sep 08 14:42:28.029 CEST
    Protocol shutdown:             No
    Non-stop forwarding:           Disabled
    Most recent startup mode:      Cold Restart

  Topologies supported by IS-IS
    IPv4 Unicast
      Distance:                    115
      Rib:                         connected
      Advertising local prefixes:  Passive Interfaces Only
      Redistributing:              no protocols
      Level-1
        Metric style:              Wide/Wide (generate/accept)
        Default Interface Metric:  10
        Advertising prefixes:      26
          Connected:               1
          Propagated:              25
            Warning when over:     40
          Redistributed:           0
            Limit:                 10000
            Warning when over:     7000 (70%)
      Level-2
        Metric style:              Wide/Wide (generate/accept)
        Default Interface Metric:  10
        Advertising prefixes:      61
          Connected:               1
          Propagated:              60
            Limit:                 60 (exceeded)
            Warning when over:     30 (50%)
          Redistributed:           0
            Limit:                 10000
            Warning when over:     7000 (70%)
    IPv6 Unicast
      Distance:                    115
      Rib:                         connected
      Advertising local prefixes:  Passive Interfaces Only
      Redistributing:              no protocols
      Level-1
        Default Interface Metric:  10
        Advertising prefixes:      1
          Connected:               1
          Propagated:              0
            Warning when over:     10
          Redistributed:           0
            Limit:                 10000
            Warning when over:     7000 (70%)
      Level-2
        Default Interface Metric:  10
        Advertising prefixes:      18
          Connected:               1
          Propagated:              17
            Warning when over:     10 (exceeded)
          Redistributed:           0
            Limit:                 10000
            Warning when over:     7000 (70%)
  Overload Bit:                    not configured
  Maximum Metrics:                 not configured

  LSP MTU:                         1492
  LSP Full:                        level-1: No, level-2: No
  LSP fast flooding:               Disabled
    Max LSP transmit speed:        1000 LSPs/sec
    PSNP delay:                    500 ms
    PSNP interval:                 50 ms

  SR-MPLS:
    SRLB allocated:                15000 - 15999
    SRGB (index 0) notified:       never
    SRGB (index 0):                allocated, 16000 - 23999

  MFI LSD registered:              Yes
    MFI LSD last registered:       Tue 2026 Sep 08 14:42:32.808 CEST
  TE connection status:            Up
    TE connection up:              Tue 2026 Sep 08 14:42:38.829 CEST
    TE connection flaps:           0
  XTC connection status:           Up
    XTC connection up:             Tue 2026 Sep 08 14:42:38.827 CEST
    XTC connection flaps:          0
  BFD connection status:           Up
    BFD connection up:             Tue 2026 Sep 08 14:42:27.600 CEST
    BFD connection flaps:          0
    BFD HA role change:            never
      BFD client HA role:          none
      BFD server HA role:          none

  System memory state:             Normal
    Last state change:             never
  IS-IS memory state:              Normal
    Current IS-IS memory usage:    22073984 bytes (1%)
    Limit (libmemdbg):             1610612400
    Last state change:             never
    Usage during process start:    733936 bytes (0%)
    Monitoring interval:           60 sec
  IS-IS memory usage per item:     items    bytes
    Nodes:                         7        2968
    LSPs:                          10       4454
    Links:                         28       34048
    Prefixes:                      380      370912
    Nets:                          139      108976
    Paths:                         334      32064

  Interfaces supported by IS-IS eu:
    Loopback0 is running passively (passive in configuration)
    GigabitEthernet0/0/0/35 is running actively (active in configuration)
    GigabitEthernet0/0/0/45 is running actively (active in configuration)
    GigabitEthernet0/0/0/56 is running actively (active in configuration)
    GigabitEthernet0/0/0/65 is running actively (active in configuration)

    Review the advertised prefix counts and configured limits for each level.

  4. Execute the show isis route detail command to display a route that is not advertised because the propagation limit was exceeded.

    Example:

    Router# show isis route 2.4.1.197/32 detail 
Mon Aug 24 12:54:55.932 CEST

L1 2.4.1.197/32 [110/115] Label: None, medium priority
   Installed Aug 24 12:48:46.944 for 00:06:10
     via 1.1.45.4, GigabitEthernet0/0/0/45, brat, SRGB Base: 16000, Weight: 0
     src brat.00-02, 1.1.100.4
   L2 adv [110] native, propagated, over limit: not advertised
Router#

IS-IS generates warnings when the propagated prefix count exceeds the configured threshold, and stops advertising additional propagated routes once the hard limit is reached. If only a warning threshold is set, IS-IS continues advertising routes but generates notifications.

What to do next

Monitor route propagation levels using router logs and the show isis protocol command to ensure limits are effective.