IP Routing: ISIS Configuration Guide, Cisco IOS Release 12.4T
Overview of IS-IS Fast Convergence
Downloads: This chapterpdf (PDF - 78.0 KB) The complete bookPDF (PDF - 1.07 MB) | Feedback

Overview of IS-IS Fast Convergence

Overview of IS-IS Fast Convergence

Last Updated: January 30, 2012

This module provides information about the topics of Intermediate System-to-Intermediate System (IS-IS) fast convergence. The tasks in the modules that follow this overview can help you improve convergence times for IS-IS networks.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Prerequisites for IS-IS Fast Convergence

You should be familiar with the concepts described in the "Integrated IS-IS Routing Protocol Overview" module.

Information About IS-IS Fast Convergence

Network Convergence

Convergence is the process of all routers coming to agreement on optimal routes in a network. When a network event causes routes to become available or unavailable, routers send routing update messages through the network that cause routing algorithms to recalculate optimal routes. Eventually all the routers agree on the routes as well as the network topology. Fast convergence benefits network performance. Routing algorithms that converge slowly may cause temporary routing loops or temporary network unavailability.

The process of network convergence can be divided into three separate stages:

  1. Routing change detection: The speed at which a device on the network can detect and react to the failure or modification of one of its own components, or to a topology change caused by the failure or modification of a component on a routing protocol peer.
  2. Routing change notification: The speed at which the failure or topology change in the previous stage can be communicated to other devices in the network.
  3. Alternate path calculation: The speed at which all devices on the network, having been notified of the failure or topology change, can process the information and calculate an alternate path through which data can flow.

An improvement in any one of these stages provides an improvement in overall convergence. In addition to a basic configuration task that is recommended as a first step in configuring an IS-IS router with best practice parameters for achieving fast convergence, several recommended configuration tasks are grouped according to the stage of network convergence they can improve. For more information, see the following modules:

  • "Setting Best Practice Parameters for IS-IS Fast Convergence"
  • "Reducing Failure Detection Times in IS-IS Networks"
  • "Reducing Link Failure and Topology Change Notification Times in IS-IS Networks"
  • "Reducing Alternate-Path Calculation Times in IS-IS Networks"

Design Recommendations for Achieving Faster Network Convergence

A faster processor can provide better performance for network convergence.

On some Cisco routers such as the Cisco 12000, 10000, 7600 and 6500 series Internet routers, the control-plane and forwarding-plane are separated. Tasks associated with network convergence such as shortest path first (SPF) calculation, routing table updates, and server functions for information distribution to line cards are supported separately from packet forwarding tasks. By leveraging the separated control-plane CPU, network convergence tasks are handled more efficiently.


For the Cisco 12000 series Internet routers, we recommend that you when you configure the process-max-time command, do not use a value lower than 60 milliseconds.

Where to Go Next

To configure features to improve IS-IS network convergence times, complete the optional tasks in one or more of the following modules:

  • "Setting Best Practice Parameters for IS-IS Fast Convergence"
  • "Reducing Failure Detection Times in IS-IS Networks"
  • "Reducing Link Failure and Topology Change Notification Times in IS-IS Networks"
  • "Reducing Alternate-Path Calculation Times in IS-IS Networks"

To enhance IS-IS network security, see the "Enhancing Security in an IS-IS Network" module.

Additional References

Related Documents

Related Topic

Document Title

IS-IS commands: complete command syntax, command mode, defaults, command history, usage guidelines, and examples

Cisco IOS IP Routing: ISIS Command Reference

Overview of Cisco IS-IS conceptual information with links to all the individual IS-IS modules

"Integrated IS-IS Routing Protocol Overview" module




ISO 8473

CLNP, Connectionless Network Protocol

ISO 9542

ES-IS Routing Information Exchange Protocol

ISO/IEC 10589

IS-IS Protocol



MIBs Link

No new or modified MIBs are supported, and support for existing MIBs has not been modified.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:







Technical Assistance



The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.


Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S. and other countries. To view a list of Cisco trademarks, go to this URL: www.cisco.com/go/trademarks. Third-party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1110R)

Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

© 2012 Cisco Systems, Inc. All rights reserved.