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

This module contains the following sections:

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.
  • When you use the system mtu bytes command to change the system MTU size, you must reset the switch before the new configuration takes effect. The system mtu routing command does not require a switch reset to take effect.

The system MTU setting is saved in the switch environmental variable in NVRAM and becomes effective when the switch reloads. Unlike the system MTU routing configuration, the MTU settings you enter with the system mtu and system mtu jumbo commands are not saved in the switch Cisco IOS configuration file, even if you enter the copy running-config startup-config privileged EXEC command. Therefore, if you use TFTP to configure a new switch by using a backup configuration file and want the system MTU to be other than the default, you must explicitly configure the system mtu and system mtu jumbo settings on the new switch and then reload the switch.

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 mtucommand 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 mtubytes 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 mtubytes 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 ActionPurpose
    Step 1configure terminal


    Example:
    Switch# configure terminal
     

    Enters global configuration mode.

     
    Step 2system 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 3end


    Example:
    Switch(config)#  end
     

    Returns to privileged EXEC mode.

     
    Step 4copy running-config startup-config


    Example:
    Switch# copy running-config startup-config
     

    Saves your entries in the configuration file.

     
    Step 5reload


    Example:
    Switch#  reload
     

    Reloads the operating system.

     
    Step 6show 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 ActionPurpose
      Step 1configure terminal


      Example:
      Switch# configure terminal
       

      Enters global configuration mode.

       
      Step 2interface interface


      Example:
      Switch(config)# interface gigabitethernet0/0
       

      Enters interface configuration mode.

       
      Step 3ip mtu bytes


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

      (Optional) Changes the IPv4 MTU size

       
      Step 4ipv6 mtu bytes


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

      (Optional) Changes the IPv6 MTU size.

       
      Step 5end


      Example:
      Switch(config-if)#  end
       

      Returns to privileged EXEC mode.

       
      Step 6copy running-config startup-config


      Example:
      Switch# copy running-config startup-config
       

      Saves your entries in the configuration file.

       
      Step 7reload


      Example:
      Switch#  reload
       

      Reloads the operating system.

       
      Step 8show 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)# exit
Switch# reload

      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.

      Additional References for System MTU

      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

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      http:/​/​www.cisco.com/​support