Table Of Contents

Overview of IS-IS Fast Convergence

Contents

Prerequisites for IS-IS Fast Convergence

Information About IS-IS Fast Convergence

Network Convergence

Design Recommendations for Achieving Faster Network Convergence

Where to Go Next

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Overview of IS-IS Fast Convergence

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First Published: November 30, 2007

Last Updated: November 30, 2007

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.

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Contents

•Prerequisites for IS-IS Fast Convergence

•Information About IS-IS Fast Convergence

•Where to Go Next

•Additional References

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

You should understand the following concepts before you configure any features to improve IS-IS network convergence times:

•Network Convergence

•Design Recommendations for Achieving Faster Network 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.

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Note 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.

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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

The following sections provide references related to IS-IS configuration tasks to achieve fast convergence.

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 Protocols Command Reference
Roadmap of IS-IS features	"Integrated IS-IS Features Roadmap" module
Overview of Cisco IS-IS conceptual information with links to all the individual IS-IS modules	"Integrated IS-IS Routing Protocol Overview" module

Standards

Standard	Title
ISO 8473	CLNP, Connectionless Network Protocol
ISO 9542	ES-IS Routing Information Exchange Protocol
ISO/IEC 10589	IS-IS Protocol

MIBs

MIB	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: http://www.cisco.com/go/mibs

RFCs

RFC	Title
None	—

Technical Assistance

Description

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