Configuring Fast Switching

Last Updated: November 3, 2011

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 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.

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 Action Purpose
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 Action Purpose
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 Action Purpose
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 Action Purpose
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 Action Purpose
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 Action Purpose
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 Action Purpose
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 Action Purpose
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 Action Purpose
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.

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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).

Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at 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. (1005R)

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.

© 2011 Cisco Systems, Inc. All rights reserved.