IP Switching: Configuring Fast Switching Configuration Guide, Cisco IOS Release 15S
Configuring Fast Switching
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Configuring Fast Switching

Contents

Configuring Fast Switching

This module describes how to configure fast switching on Cisco IOS devices and provides configuration guidelines for switching paths and tuning guidelines.


Note


IP unicast fast switching is no longer supported on Cisco IOS Releases 12.2(25)S, 12.2(28)SB, 12.2(33)SRA, 12.2(33)SXH, 12.4(20)T and later releases. For these and later releases, components that do not support Cisco Express Forwarding will only work in Process Switched mode.


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.

Information About Configuring Fast Switching

Benefits of Fast Switching

Fast switching allows higher throughput by switching a packet using a cache created by the initial packet sent to a particular destination. Destination addresses are stored in the high-speed cache to expedite forwarding. Routers offer better packet-transfer performance when fast switching is enabled. Fast switching is enabled by default on all interfaces that support fast switching.

When packets are fast switched, the first packet is copied to packet memory and the destination network or host is found in the fast-switching cache. The frame is rewritten and sent to the outgoing interface that services the destination. Subsequent packets for the same destination use the same switching path. The interface processor computes the CRC.


Note


IP unicast fast switching is no longer supported on Cisco IOS Releases 12.2(25)S, 12.2(28)SB, 12.2(33)SRA, 12.2(33)SXH, 12.4(20)T and later releases. For these and later releases, components that do not support Cisco Express Forwarding will only work in Process Switched mode.


Reasons for Disabling Fast Switching

Fast switching uses a cache created by previous packets to achieve a higher packet throughput. Packet transfer performance is generally better when fast switching is enabled. Fast switching also provides load sharing on a per-destination basis.

By default, fast switching is enabled on all interfaces that support fast switching. However, you may want to disable fast switching to save memory space on interface cards and to help avoid congestion when high-bandwidth interfaces are writing large amounts of information to low-bandwidth interfaces. This is especially important when using rates slower than T1.

Fast switching is not supported on serial interfaces using encapsulations other than HDLC.


Note


Turning off fast switching increases system overhead because the packets are then process switched by the system’s CPU.


For some diagnostics, such as debugging and packet-level tracing, you need to disable fast switching. Disabling fast switching causes the router to fall back to process switching the packets. If fast switching is running, you might only see the first packet to each destination in the output of any packet-level debugging commands. Subsequent packets to the same destination are fast switched. Many packet level debugging commands cannot process packets that are fast switched. You might want to turn off fast switching temporarily to use process switching instead while you are trying to capture information to diagnose a problem.

AppleTalk Access Lists Automatically Fast Switched

AppleTalk access lists are automatically fast switched. Access list fast switching improves the performance of AppleTalk traffic when access lists are defined on an interface.

Refer to the "Configuring AppleTalk" chapter in the Cisco IOS AppleTalk and Novell IPX Configuration Guidefor guidelines on creating and using access lists and configuring AppleTalk.

How to Configure Fast Switching

By default, fast switching is enabled on all interfaces that support fast switching. However, you may have reasons to disable fast switching (see the Reasons for Disabling Fast Switching).

The tasks in this section include enabling fast switching for some software applications, disabling fast switching for other software applications, and managing the route cache associated with fast switching on the device:


Note


Fast switching is not supported for the X.25 encapsulations.


Enabling Fast Switching of IPX Directed Broadcast Packets

To enable fast switching of Internet Packet Exchange (IPX) directed broadcast packets, perform the following task. This may be useful in certain broadcast-based applications that rely on helpering.

By default, Cisco IOS software switches IPX packets that are directed to the broadcast address. Fast switching of these packets is disabled. The default behavior is to process switch directed broadcast packets.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    ipx broadcast-fastswitching

    4.    end


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 ipx broadcast-fastswitching


    Example:
    Router(config)# ipx broadcast-fastswitching
     

    Enables the router to fast switch IPX directed broadcast packets,

     
    Step 4 end


    Example:
    Router(config)# end
     

    Exits to privileged EXEC mode.

     

    Disabling IPX Fast Switching

    To disable IPX fast switching, perform the following task. IPX fast switching is enabled by default. You might want to disable fast switching for the following reasons:

    • To save memory on the interface cards: fast-switching caches require more memory than those used for standard switching
    • To avoid congestion on interface cards when a high-bandwidth interface is writing large amounts of information to a low-bandwidth interface
    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    interface type number

      4.    no ipx route-cache

      5.    end


    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 interface type number


      Example:
      Router(config)# interface ethernet 0
       

      Configures an interface type and enters interface configuration mode.

      • The type argument is the type of interface to be configured.
      • The number argument is the port, connector, or interface card number. The numbers are assigned at the factory at the time of installation or when added to a system, and can be displayed with the show interfacescommand.
       
      Step 4 no ipx route-cache


      Example:
      Router(config-if)# no ipx route-cache
       

      Disables IPX fast switching on an interface.

       
      Step 5 end


      Example:
      Router(config)# end
       

      Exits to privileged EXEC mode.

       

      Adjusting the Route Cache for IPX

      Adjusting the route cache allows you to control the size of the route cache, reduce memory consumption, and improve router performance. You accomplish these tasks by controlling the route cache size and route cache invalidation. The following sections describe these optional tasks:

      Controlling IPX Route Cache Size

      You can limit the number of entries stored in the IPX route cache to free up router memory and aid router processing.

      Storing too many entries in the route cache can use a substantial amount of router memory, causing router processing to slow. This situation is most common on large networks that run network management applications for NetWare.

      For example, if a network management station is responsible for managing all clients and servers in a very large (greater than 50,000 nodes) Novell network, the routers on the local segment can become inundated with route cache entries. You can set a maximum number of route cache entries on these routers to free up router memory and aid router processing.

      To control IPX route cache size, perform the following task.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    ipx route-cache max-size size

        4.    end


      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 ipx route-cache max-size size


        Example:
        Router(config)# ipx route-cache max-size 10000
         

        Sets a maximum limit on the number of entries in the IPX route cac he.

        • The size argument is maximum number of entries allowed in the IPX route cache.
        Note   

        If the route cache has more entries than the specified limit, the extra entries are not deleted. However, they may be removed if route cache invalidation is in use. See the Controlling IPX Route Cache Entry Invalidation" for more information on invalidating route cache entries.

         
        Step 4 end


        Example:
        Router(config)# end
         

        Exits to privileged EXEC mode.

         

        Controlling IPX Route Cache Entry Invalidation

        You can configure the router to invalidate inactive fast-switch cache entries. If these entries remain invalidated for 1 minute, the router purges the entries from the route cache.

        Purging invalidated entries reduces the size of the route cache, reduces memory consumption, and improves router performance. Purging entries also helps ensure accurate route cache information.

        You specify the period of time that valid fast switch cache entries must be inactive before the router invalidates them. You can also specify the number of cache entries that the router can invalidate per minute.

        To control IPX route cache entry invalidation, perform the following task.

        SUMMARY STEPS

          1.    enable

          2.    configure terminal

          3.    ipx route-cache inactivity-timeout period [rate]

          4.    end


        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 ipx route-cache inactivity-timeout period [rate]


          Example:
          Router(config)# ipx route-cache inactivity-timeout 5 10
           

          Adjusts the period and rate of route cache invalidation because of inactivity.

          • The period argument is the number of minutes that a valid cache entry may be inactive before it is invalidated. Valid values are 0 through 65,535. A value of zero disables this feature. The default is 2.
          • The rate argument is the maximum number of inactive entries that may be invalidated per minute. Valid values are 0 through 65,535. The default rate is 0 (cache entries do not age).
          Note   

          When you use the ipx route-cache inactivity-timeout command with the ipx route-cache max-size command, you can ensure a small route cache with fresh entries.

           
          Step 4 end


          Example:
          Router(config)# end
           

          Exits to privileged EXEC mode.

           

          Enabling Padding of Odd-Length IPX Packets

          Some IPX end hosts accept only even-length Ethernet packets. If the length of a packet is odd, the packet must be padded with an extra byte so that end host can receive it. By default, Cisco IOS software pads odd-length Ethernet packets.


          Note


          However, there are cases in certain topologies where nonpadded Ethernet packets are forwarded onto a remote Ethernet network. Under specific conditions, you can enable padding on intermediate media as a temporary workaround for this problem. Note that you should perform this task only under the guidance of a customer engineer or other service representative.


          To enable the padding of odd-length packets, perform the following task.

          SUMMARY STEPS

            1.    enable

            2.    configure terminal

            3.    interface type number

            4.    no ipx route-cache

            5.    ipx pad-process-switched-packets

            6.    end


          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 interface type number


            Example:
            Router(config)# interface serial 0
             

            Configures an interface type and enters interface configuration mode.

            • The type argument is the type of interface to be configured.
            • The number argument is the port, connector, or interface card number. The numbers are assigned at the factory at the time of installation or when added to a system, and can be displayed with the show interfacescommand.
             
            Step 4 no ipx route-cache


            Example:
            Router(config-if)# no ipx route-cache
             

            Disables IPX fast switching

             
            Step 5 ipx pad-process-switched-packets


            Example:
            Router(config-if)# ipx pad-process-switched-packets
             

            Controls whether odd-length packets are padded so as to be sent as even-length packets on an interface.

             
            Step 6 end


            Example:
            Router(config-if)# end
             

            Exits to privileged EXEC mode.

             

            Disabling AppleTalk Fast Switching

            To disable AppleTalk fast switching on an interface, perform the following task. AppleTalk fast switching is enable by default.

            See the Reasons for Disabling Fast Switching for information on when you might want to disable AppleTalk fast switching.

            SUMMARY STEPS

              1.    enable

              2.    configure terminal

              3.    interface type number

              4.    no appletalk route-cache

              5.    end


            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 interface type number


              Example:
              Router(config)# interface ethernet 0
               

              Configures an interface type and enters interface configuration mode.

              • The type argument is the type of interface to be configured.
              • The number argument is the port, connector, or interface card number. The numbers are assigned at the factory at the time of installation or when added to a system, and can be displayed with the show interfacescommand.
               
              Step 4 no appletalk route-cache


              Example:
              Router(config-if)# no appletalk route-cache
               

              Disables AppleTalk fast switching.

               
              Step 5 end


              Example:
              Router(config-if)# end
               

              Exits to privileged EXEC mode.

               

              Reenabling SMDS Fast Switching for IPX and AppleTalk Packets

              Switched Multimegabit Data Service (SMDS) fast switching is enabled by default. To reenable SMDS fast switching on IPX and AppleTalk packets, if it has been disabled, perform the following task.

              SMDS is a wide-area networking service offered by some Regional Bell Operating Companies (RBOCs). SMDS fast switching of IPX and AppleTalk packets provides faster packet transfer on serial links with speeds above 56 kbps. Use fast switching if you use high-speed, packet-switched, datagram-based WAN technologies such as Frame Relay offered by service providers.

              SUMMARY STEPS

                1.    enable

                2.    configure terminal

                3.    interface type number

                4.    encapsulation smds

                5.    ipx route-cache

                6.    appletalk route-cache

                7.    end


              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 interface type number


                Example:
                Router(config)# interface serial 0
                 

                Configures an interface type and enters interface configuration mode.

                • The type argument is the type of interface to be configured.
                • The number argument is the port, connector, or interface card number. The numbers are assigned at the factory at the time of installation or when added to a system, and can be displayed with the show interfacescommand.
                 
                Step 4 encapsulation smds


                Example:
                Router(config-if)# encapsulation smds
                 

                Enables SMDS on the desired interface.

                 
                Step 5 ipx route-cache


                Example:
                Router(config-if)# ipx route-cache
                 

                Enables IPX fast switching on the interface.

                 
                Step 6 appletalk route-cache


                Example:
                Router(config-if)# appletalk route-cache
                 

                Enables AppleTalk fast switching on all supported interfaces.

                 
                Step 7 end


                Example:
                Router(config-if)# end
                 

                Exits to privileged EXEC mode.

                 

                Disabling DECnet Fast Switching

                To disable fast switching of DECnet packets, perform the following task.

                By default, DECnet routing software implements fast switching of DECnet packets. You might want to disable fast switching to save memory space on interface cards and to help avoid congestion when high-bandwidth interfaces are writing large amounts of information to low-bandwidth interfaces. Disabling fast switching is especially important when rates slower than T1 are used.

                SUMMARY STEPS

                  1.    enable

                  2.    configure terminal

                  3.    interface type number

                  4.    no decnet route-cache

                  5.    end


                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 interface type number


                  Example:
                  Router(config)# interface serial 0/0
                   

                  Configures an interface type and enters interface configuration mode.

                  • The type argument is the type of interface to be configured.
                  • The number argument is the port, connector, or interface card number. The numbers are assigned at the factory at the time of installation or when added to a system, and can be displayed with the show interfacescommand.
                   
                  Step 4 no decnet route-cache


                  Example:
                  Router(config-if)# no decnet route-cache
                   

                  Disables fast switching of DECnet packets on a per-interface basis.

                   
                  Step 5 end


                  Example:
                  Router(config-if)# end
                   

                  Exits to privileged EXEC mode.

                   

                  Disabling ISO CLNS Fast Switching Through the Cache

                  Perform the following task to disable See the Reasons for Disabling Fast Switching for information on why you might want to disable ISO CLNS fast switching through the cache.

                  SUMMARY STEPS

                    1.    enable

                    2.    configure terminal

                    3.    interface type number

                    4.    no clns route-cache

                    5.    end


                  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 interface type number


                    Example:
                    Router(config)# interface ethernet 0
                     

                    Configures an interface type and enters interface configuration mode.

                    • The type argument is the type of interface to be configured.
                    • The number argument is the port, connector, or interface card number. The numbers are assigned at the factory at the time of installation or when added to a system, and can be displayed with the show interfacescommand.
                     
                    Step 4 no clns route-cache


                    Example:
                    Router(config-if)# no clns route-cache
                     

                    Disables fast switching.

                    Note   

                    The cache still exists and is used after the no clns route-cache command is entered, but the software does not do fast switching through the cache.

                     
                    Step 5 end


                    Example:
                    Router(config-if)# end
                     

                    Exits to privileged EXEC mode.

                     

                    Configuration Examples for Fast Switching

                    Example Enabling Fast Switching of IPX Directed Broadcast Packets

                    The following example shows how to enable fast switching of IPX directed broadcast packets:

                    configure terminal
                    ipx broadcast-fastswitching
                    end

                    Example Disabling IPX Fast Switching

                    The following example shows how to disable IPX fast switching:

                    configure terminal
                    interface ethernet 0
                     no ipx route-cache
                     end

                    Example Adjusting the Route Cache for IPX

                    The following examples show how to adjust the route cache for IPX. This allows you to control the size of the route cache, reduce memory consumption, and improve router performance.

                    Example Controlling IPX Route Cache Size

                    The following example show how to control the IPX route cache size:

                    configure terminal
                    ipx route-cache max-size 10000
                    end
                    

                    In this example the cache size is set to 10000 entries. If the route cache has more entries than the specified limit, the extra entries are not deleted. However, they may be removed if route cache invalidation is in use. See the "Example Controlling IPX Route Cache Entry Invalidation"for a configuration example.

                    Example Controlling IPX Route Cache Entry Invalidation

                    The following example shows how to control IPX route cache entry invalidations;

                    configure terminal
                    ipx route-cache inactivity-timeout 5 10
                    end
                    

                    In this example, the inactivity period is set to 5 minutes and sets a maximum of 10 entries that can be invalidated per minute.

                    When you use the ipx route-cache inactivity-timeout command with the ipx route-cache max-size command, you can ensure a small route cache with fresh entries.

                    Example Enabling Padding of Odd-Length IPX Packets


                    Note


                    Use the ipx pad-process-switched-packets command only under the guidance of a customer engineer or other service representative.


                    The following example shows how to enable padding of odd-length IPX packets:

                    configure terminal
                    interface serial 0
                     no ipx route-cache
                     ipx pad-process-switched-packets
                     end
                    

                    In this example, the Cisco IOS software pads odd-length packets so that they are sent as even-length packets on serial interface 0.

                    Example Disabling AppleTalk Fast Switching

                    The following example shows how to disable AppleTalk fast switching:

                    configure terminal
                    interface ethernet 0
                     no appletalk route-cache
                     end

                    Example Reenabling SMDS Fast Switching for IPX and AppleTalk Packets

                    The following example shows how to reenable SMDS fast switching for IPX and AppleTalk packets if fast switching is disabled:

                    configure terminal
                    interface serial 0
                     encapsulation smds
                     ipx route-cache
                     appletalk route-cache
                     end

                    Example Disabling DECnet Fast Switching

                    The following example show how to disable DECnet fast switching:

                    configure terminal
                    interface serial 0/0
                     no decnet route-cache
                     end
                    

                    DECnet fast switching is disabled on a per-interface basis.

                    Example Disabling ISO CLNS Fast Switching Through the Cache

                    The following example shows how to disable ISO CLNS fast switching through the cache:

                    configure terminal
                    interface ethernet 0
                     no clns route-cache
                     end

                    Additional References

                    Related Documents

                    Related Topic

                    Document Title

                    Cisco IOS commands

                    Cisco IOS Master Commands List, All Releases

                    IP switching commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples.

                    Cisco IOS IP Switching Command Reference

                    Overview of switching paths available on Cisco IOS devices

                    Cisco IOS Switching Paths Overview

                    Information on how to configure AppleTalk

                    Cisco IOS AppleTalk Configuration Guide

                    Description of AppleTalk commands

                    Cisco IOS AppleTalk Command Reference

                    Information on how to configure Novell IPX

                    Cisco IOS Novell IPX Configuration Guide

                    Description of the IPX commands

                    Cisco IOS Novell IPX Command Reference

                    Information on how to configure SMDS packet-switched software

                    "Configuring SDMS " chapter in the Access and Communication Servers Configuration Guide

                    Description of SMDS commands

                    "SMDS Commands " chapter in the Access and Communication Servers Command Reference

                    Information on how to configure DECnet

                    Cisco IOS DECnet Configuration Guide

                    Description of DECnet command

                    Cisco IOS DECnet Command Reference

                    Information on how to configure ISO CLNS

                    Cisco IOS ISO CLNS Configuration Guide

                    Description of ISO CLNS commands

                    Cisco IOS ISO CLNS Command Reference

                    Standards

                    Standard

                    Title

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

                    --

                    MIBs

                    MIB

                    MIBs Link

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

                    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

                    No new or modified RFCs are supported by this feature, and support for existing RFCs 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 Configuring Fast Switching

                    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 Configuring Fast Switching

                    Feature Name

                    Releases

                    Feature Information

                    This table is intentionally left blank because no features were introduced or modified in Cisco IOS Release 12.2(1) or later. This table will be updated when feature information is added to this module.

                    --

                    --

                    Glossary

                    AppleTalk --A multilayered protocol providing internetwork routing, transaction and data stream service, naming service, and comprehensive file and print sharing.

                    IPX --Internetwork Packet Exchange. A NetWare protocol that routes outgoing data packets across a network. Every NetWare network has a unique address assigned when its servers are configured. IPX routers use this address to route packets through an internetwork.

                    ISO CLNS --International Organization for Standardization (ISO) Connectionless Network Service (CLNS). A standard for the network layer of the Open System Interconnection (OSI) model. CLNS is the OSI network layer service that does not require a circuit to be established before data is transmitted. CLNS routes messages to their destination independently of any other message.

                    NetWare --Popular distributed network operating system developed by Novell.

                    SMDS --Switched Multimegabit Data Service. A wide-area networking service offered by some Regional Bell Operating Companies (RBOCs).