Interface and Hardware Component Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850 Switches)
Configuring System MTU
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Configuring System MTU

Configuring System MTU

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.

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

Information about the MTU

The default maximum transmission unit (MTU) size for frames received and sent on all switch interfaces is 1500 bytes. You can change the MTU size to support switched jumbo frames on all Gigabit Ethernet and 10-Gigabit Ethernet interfaces and to support routed frames on all routed ports.

System MTU Values

The following MTU values can be configured:

  • System MTU--This value applies to switched packets on the Gigabit Ethernet and 10-Gigabit Ethernet ports of the switch. Use the system mtu bytes global configuration command to specify the system jumbo MTU value.
  • Protocol-specific MTU--This value applies only to routed packets on all routed ports of the switch or switch stack. Use the ip mtu bytes or ipv6 mtu bytes interface configuration command to specify the protocol-specific MTU value.

System MTU Guidelines

When configuring the system MTU values, follow these guidelines:

  • The switch does not support the MTU on a per-interface basis.
  • If you enter the system mtu bytes global configuration command, the command does not take effect on the switch. This command only affects the system MTU size on Fast Ethernet switch ports.

System MTU Value Application

This table shows how the MTU values are applied.

Table 1  MTU Values

Configuration

system mtu command

ip mtu command

ipv6 mtu command

Standalone switch or switch stack

You can enter the system mtu command on a switch or switch stack, but system MTU value does not take effect.

The range is from 1500 to 9198 bytes.

Use the ip mtu bytes command.

The range is from 68 up to the system MTU value (in bytes).
Note   

The IP MTU value is the applied value, not the configured value.

Use the ipv6 mtu bytes command.

The range is from 1280 to the system jumbo MTU value (in bytes).
Note   

The IPv6 MTU value is the applied value, not the configured value.

The upper limit of the IP or IPv6 MTU value is based on the switch or switch stack configuration and refers to the currently applied system MTU value. For more information about setting the MTU sizes, see the system mtu global configuration command in the command reference for this release.

How to Configure MTU Sizes

Configuring the System MTU

Beginning in privileged EXEC mode, follow these steps to change the MTU size for switched packets:

SUMMARY STEPS

    1.    configure terminal

    2.    system mtu bytes

    3.    end

    4.    copy running-config startup-config

    5.    reload

    6.    show system mtu


DETAILED STEPS
      Command or Action Purpose
    Step 1 configure terminal


    Example:
    Switch# configure terminal
     

    Enters global configuration mode.

     
    Step 2 system mtu bytes


    Example:
    Switch(config)# system mtu 7500
     

    (Optional) Changes the MTU size for all Gigabit Ethernet and 10-Gigabit Ethernet interfaces on the switch or the switch stack.

     
    Step 3 end


    Example:
    Switch(config)#  end
     

    Returns to privileged EXEC mode.

     
    Step 4 copy running-config startup-config


    Example:
    Switch# copy running-config startup-config
     

    Saves your entries in the configuration file.

     
    Step 5 reload


    Example:
    Switch#  reload
     

    Reloads the operating system.

     
    Step 6 show system mtu


    Example:
    Switch# show system mtu
     

    Verifies your settings.

     

    Configuring Protocol-Specific MTU

    Beginning in privileged EXEC mode, follow these steps to change the MTU size for routed ports:

    SUMMARY STEPS

      1.    configure terminal

      2.    interface interface

      3.    ip mtu bytes

      4.    ipv6 mtu bytes

      5.    end

      6.    copy running-config startup-config

      7.    reload

      8.    show system mtu


    DETAILED STEPS
        Command or Action Purpose
      Step 1 configure terminal


      Example:
      Switch# configure terminal
       

      Enters global configuration mode.

       
      Step 2 interface interface


      Example:
      Switch(config)# interface gigabitethernet0/0
       

      Enters interface configuration mode.

       
      Step 3 ip mtu bytes


      Example:
      Switch(config-if)# ip mtu 68
       

      (Optional) Changes the IPv4 MTU size

       
      Step 4 ipv6 mtu bytes


      Example:
      Switch(config-if)# ipv6 mtu 1280
       

      (Optional) Changes the IPv6 MTU size.

       
      Step 5 end


      Example:
      Switch(config-if)#  end
       

      Returns to privileged EXEC mode.

       
      Step 6 copy running-config startup-config


      Example:
      Switch# copy running-config startup-config
       

      Saves your entries in the configuration file.

       
      Step 7 reload


      Example:
      Switch#  reload
       

      Reloads the operating system.

       
      Step 8 show system mtu


      Example:
      Switch# show system mtu
       

      Verifies your settings.

       

      Configuration Examples for System MTU

      This example shows how to set the maximum packet size for a Gigabit Ethernet port to 7500 bytes:

      Switch(config)# system mtu 7500
      Switch(config)# 
      Switch(config)# exit
      
      
      

      If you enter a value that is outside the allowed range for the specific type of interface, the value is not accepted. This example shows the response when you try to set Gigabit Ethernet interfaces to an out-of-range number:

      Switch(config)# system mtu 25000
                                         ^
      % Invalid input detected at '^' marker.
      
      

      This is an example of output from the show system mtu command:

      Switch# show system mtu
      Global Ethernet MTU is 1500 bytes.
      
      

      Additional References for System MTU

      Error Message Decoder

      Description Link

      To help you research and resolve system error messages in this release, use the Error Message Decoder tool.

      https:/​/​www.cisco.com/​cgi-bin/​Support/​Errordecoder/​index.cgi

      MIBs

      MIB MIBs Link

      All supported MIBs for this release.

      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

      Technical Assistance

      Description Link

      The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

      To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

      Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

      http:/​/​www.cisco.com/​support

      Feature Information for System MTU

      Release

      Modification

      Cisco IOS XE 3.2SE

      This feature was introduced.