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

LSP flood limits

Updated: June 9, 2026

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Explains LSP flood limitation strategies for IS-IS, including configuration procedures for LSP flooding parameters and the use of LSP fast flood control mechanisms to prevent excessive network load.

A flood limit is an IS-IS network design technique that

  • restricts the flooding of link-state packets (LSPs) on specific interfaces or mesh groups

  • improves network scalability in highly redundant or meshy topologies, and

  • may reduce network reliability in the event of multiple failures.

Use cases for limiting LSP flooding

Limiting link-state packets (LSPs) may be desirable in certain “meshy” network topologies, such as highly redundant networks with a fully meshed set of point-to-point links over a nonbroadcast multiaccess (NBMA) transport. In these networks, full LSP flooding can limit network scalability.

Techniques for restricting LSP flooding

One way to restrict the size of the flooding domain is to introduce hierarchy with multiple Level 1 areas and a Level 2 area. Two other techniques can also be used, instead of or in addition to hierarchy:

  • Blocking flooding on specific interfaces

  • Configuring mesh groups

Consequences of flood limiting

Both blocking and mesh group techniques operate by restricting the flooding of LSPs. While scalability of the network is improved, network reliability may be reduced. A series of failures can prevent LSPs from being flooded throughout the network, even if links exist that would allow flooding if blocking or mesh groups had not restricted their use. Under these conditions, link-state databases of routers may become unsynchronized, and consequences such as persistent forwarding loops can result.

Recommendation for network design

Blocking or mesh groups should be used only if specifically required, and then only after careful network design.

LSP fast flood control mechanisms for IS-IS networks

A fast flood control mechanism is an IS-IS protocol enhancement that

  • increases the transmission rate of Link State Packets (LSPs) across IS-IS domains,

  • dynamically adjusts flooding rates based on neighbor acknowledgment processing, and

  • enables configurable parameters for flexible activation and rate control.

Table 1. Feature History Table

Feature Name

Release Name

Description

LSP Fast-Flooding on IS-IS Networks

 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.

LSP Fast-Flooding on IS-IS Networks

 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
LSP Fast-Flooding on IS-IS Networks Release 24.3.1

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*)

You can now accelerate the rate at which Link State Packets (LSPs) are distributed across an IS-IS network. Faster LSP distribution means faster network convergence. This faster convergence ensures that the most accurate topology information is quickly available across all routers on the network, reducing the chances of routing loops or misrouting.

The feature introduces these changes:

CLI:

YANG Data Model:

Cisco-IOS-XR-um-router-isis-cfg

(refer to GitHub and YANG Data Models Navigator)

*This feature is supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 88-LC1-36EH

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

Flexibility and activation

Unlike traditional IS-IS flooding, which operates with a fixed timer, fast-flooding mechanisms provide greater adaptability and control for network administrators. Adjustable parameters such as the local Partial Sequence Number PDU (PSNP) interval and maximum flooding rate can be set to meet specific network requirements. Fast-flooding is inactive by default, allowing administrators to enable it selectively where improved LSP flooding benefits the network.

Dynamic flooding rate optimization

Fast-flooding incorporates real-time adjustment of LSP transmission rates, using persistent monitoring of PSNP acknowledgments from neighbors. If acknowledgments are delayed, the flooding rate automatically decreases to prevent overloading neighbor routers. If acknowledgments are received promptly, the mechanism increases the flooding rate up to the configured maximum, enhancing speed of topology distribution. This adaptive approach ensures optimal balance between rapid network convergence and stability.

For instance, configuring fast-flooding in a large IS-IS network enables LSP bursts at a default rate of 1000 LSPs per second, far surpassing traditional rates. The feature dynamically adapts to real-time conditions, preventing congestion while accelerating topology updates.

Configure LSP flooding parameters for IS-IS

Tune IS-IS LSP flooding parameters to limit excessive flooding, reduce CPU load, and maintain stable IS-IS operations.

Flooding of IS-IS LSPs, if not controlled, can affect router scalability and lead to unnecessary resource consumption. Adjusting parameters lets you optimize IS-IS operation globally or per interface.

Before you begin

  • Ensure IS-IS is already configured on your router.

  • Decide whether you want to apply flooding parameters globally or on specific interfaces.

  • Review network requirements for refresh intervals, LSP lifetimes, and flooding behavior.

Procedure

1.

Enter IS-IS router configuration mode, and configure LSP flooding parameters globally.

  1. Set the LSP refresh interval (between LSP regenerations).

    Example:

    Router# configure
Router(config)# router isis isp
Router(config-isis)# lsp-refresh-interval 10800 level 2

    The refresh interval should always be set lower than the max-lsp-lifetime .

  2. Configure the LSP check interval (database checksum validation).

    Example:

    
Router(config-isis)# lsp-check-interval 240

    This operation uses significant CPU resources and should be configured to run infrequently.

  3. Configure LSP generation interval to reduce flooding during instability:

    Example:

    
Router(config-isis)# lsp-gen-interval maximum-wait 15 initial-wait 5

    During extended periods of network instability, repeated LSP recalculations can significantly increase CPU load on the local router. Additionally, flooding these recalculated LSPs to other intermediate systems increases network traffic and causes other routers to spend more time recalculating routes.

  4. Set LSP MTU (maximum transmission unit).

    Example:

    
Router(config-isis)# lsp-mtu 1300

  5. Set maximum LSP lifetime.

    Example:

    
Router(config-isis)# max-lsp-lifetime 11000

    This is the length of time an LSP remains in a neighbor’s database unless it is regenerated or refreshed.

  6. Configure purge of LSPs with checksum errors.

    Example:

    
Router(config-isis)# ignore-lsp-errors disable
2.

For interface-specific settings, enter interface configuration mode.

Example:


Router(config-isis)# interface HundredGigE 0/1/0/3
3.

Configure flooding thresholds and intervals per interface:

  1. Set LSP interval for interface.

    Example:

    
Router(config-isis-if)# lsp-interval 100

  2. Configure CSNP interval (broadcast CSNP packet frequency).

    Example:

    
Router(config-isis-if)# csnp-interval 30 level 1
    Tip

    • Frequent CSNPs increase router processing load.

    • Less frequent CSNPs may delay detection of differences.

  3. Set retransmit interval (wait time for acknowledgment).

    Example:

    
Router(config-isis-if)# retransmit-interval 60

  4. Set retransmit throttle interval (time between retransmissions).

    Example:

    
Router(config-isis-if)# retransmit-throttle-interval 1000

    This time is usually greater than or equal to the lsp-interval command time because the reason for lost LSPs may be that a neighboring router is busy. A longer interval gives the neighbor more time to receive transmissions.

  5. Optimize flooding in highly meshed networks.

    Example:

    
Router(config-isis-if)# mesh-group blocked
4.

(Optional) Use the show isis interface command to check the IS-IS interface state.

Example:


Router# show isis interface HundredGigE 0/1/0/1 brief
5.

(Optional) Use the show isis database command to check the IS-IS LSP database.

Example:


Router# show isis database level 1
6.

(Optional) Use the show isis lsp-log command to check the LSP log information.

Example:


Router# show isis lsp-log
7.

(Optional) Use the show isis database-log command to check the IS-IS database log information.

Example:


Router# show isis database-log level 1