IP Routing: BGP Configuration Guide, Cisco IOS Release 15S
Regex Engine Performance Enhancement
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Regex Engine Performance Enhancement

Regex Engine Performance Enhancement

The Regex Engine Performance Enhancement feature introduces a new regular expression engine that is designed to process complex regular expressions. This new regular expression engine does not replace the existing engine. The existing engine is preferred for simple regular expressions and is the default engine and in Cisco IOS software. Either engine can be selected from the command-line interface (CLI).

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and 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 module.

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 Regex Engine Performance Enhancement

The regular expression engine can be selected only under a Border Gateway Protocol (BGP) routing process in router configuration mode. So, the engine can be changed only after BGP has been enabled.

Information About Regex Engine Performance Enhancement

Regular Expression Overview

A regular expression is a pattern to match against an input string. You specify the pattern that a string must match when you compose a regular expression. Matching a string to the specified pattern is called “pattern matching.” Pattern matching either succeeds or fails.

A regular expression can be a single-character pattern or a multiple-character pattern. That is, a regular expression can be a single character that matches the same single character in the input string or multiple characters that match the same multiple characters in the input string.

Default Regular Expression Engine

The default Cisco IOS regular expression engine uses a recursive algorithm. This engine is effective but uses more system resources as the complexity of regular expressions increase. The recursive algorithm works well for simple regular expressions, but is less efficient when processing very complex regular expressions because of the backtracking that is required by the default engine to process partial matches. In some cases, CPU watchdog timeouts and stack overflow traces have occurred because of the length of time that the default engine requires to process very complex regular expressions.

New Regular Expression Engine Selection

The Regex Engine Performance Enhancement feature introduces a deterministic processing time regular expression engine in Cisco IOS software. This new engine does not replace the default regular expression engine. The new engine employs an improved algorithm that eliminates excessive back tracking and greatly improves performance when processing complex regular expressions. When the new engine is enabled, complex regular expressions are evaluated more quickly, and CPU watchdog timeouts and stack overflow traces will not occur. However, the new regular expression engine takes longer to process simple regular expressions than the default engine.

We recommend that you use the new regular expression engine if you need to evaluate complex regular expressions or if you have observed problems related to evaluating regular expressions. We recommend that you use the default regular expression engine if you use only simple regular expressions. The new engine can be enabled by entering the bgp regexp deterministic command under a BGP routing process. The default regular expression engine can be reenabled by entering the no form of this command.

How to Change the Regular Expression Engine

Selecting the New Regular Expression Engine

We recommend that you use the new regular expression engine if you need to evaluate complex regular expressions or if you have observed problems related to evaluating regular expressions. We recommend that you use the default regular expression engine if you only use simple regular expressions.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    router bgp as-number

    4.    bgp regexp deterministic

    5.    exit


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Router> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.
     
    Step 2 configure terminal


    Example:
    Router# configure terminal
     

    Enters global configuration mode.

     
    Step 3 router bgp as-number


    Example:
    Router(config)# router bgp 1
     

    Enters router configuration mode, and creates a BGP routing process.

     
    Step 4 bgp regexp deterministic


    Example:
    Router(config-router)# no bgp regexp deterministic 
     

    Configures Cisco IOS to use a deterministic regular expression engine.

    • The default regular expression engine in Cisco IOS software is nondeterministic.
    • The default engine can be restored by entering the no form of this command.
     
    Step 5 exit


    Example:
    Router(config-router)# exit
     

    Exits router configuration mode, and enters global configuration mode.

     

    Examples

    The following example configures Cisco IOS software to use the default regular expression engine:

    router bgp 1
     no bgp regexp deterministic 
    

    The following example configures Cisco IOS software to use the deterministic processing time regular expression engine:

    router bgp 1
     bgp regexp deterministic 

    Additional References

    Related Documents

    Related Topic

    Document Title

    Cisco IOS commands

    Cisco IOS Master Command List, All Releases

    BGP commands

    Cisco IOS IP Routing: BGP Command Reference

    Regular Expressions

    “Regular Expressions” appendix of the Cisco IOS Terminal Services Configuration Guide

    Standards

    Standards

    Title

    No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

    MIBs

    MIBs

    MIBs Link

    To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:

    http:/​/​www.cisco.com/​public/​sw-center/​netmgmt/​cmtk/​mibs.shtml

    RFCs

    RFCs

    Title

    No new or modified RFCs are supported by this feature, and support for existing standards has not been modified by this feature.

    Technical Assistance

    Description

    Link

    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.

    http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

    Feature Information for Regex Performance Enhancement

    The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

    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.

    Table 1 Feature Information for Regex Performance Enhancement

    Feature Name

    Releases

    Feature Information

    Regex Performance Enhancement

    12.0(26)S 12.3(4)T 12.2(22)S Cisco IOS XE 3.1.0SG

    The Regex Engine Performance Enhancement feature introduces a new regular expression engine that is designed to process complex regular expressions. This new regular expression engine does not replace the existing engine. The existing engine is preferred for simple regular expressions and is the default engine and in Cisco IOS software. Either engine can be selected from the command-line interface (CLI).

    The following command was introduced: bgp regexp deterministic.