Configuring System MTU

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Information about the MTU

The default maximum transmission unit (MTU) size for frames received and sent on all device interfaces is 1500 bytes.

Restrictions for System MTU

When configuring the system MTU values, follow these guidelines:

  • The device does not support the MTU on a per-interface basis.

  • If you enter the system mtu bytes global configuration command, the command affects all the switched and routed ports on the switch.

System MTU Value Application

In a switch stack, the MTU values applied to member switches depends upon the stack configuration. The following stack configurations are supported:

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

Follow these steps to change the MTU size for switchedand routed packets:

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. system mtu bytes
  4. end
  5. copy running-config startup-config
  6. reload
  7. show system mtu

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

system mtu bytes

Example:

Device(config)# system mtu 1900

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

Step 4

end

Example:

Device(config)#  end

Returns to privileged EXEC mode.

Step 5

copy running-config startup-config

Example:

Device# copy running-config startup-config

Saves your entries in the configuration file.

Step 6

reload

Example:

Device#  reload

Reloads the operating system.

Step 7

show system mtu

Example:

Device# show system mtu

Verifies your settings.

Configuring Protocol-Specific MTU

To override system MTU values on routed interfaces, configure protocol-specific MTU under each routed interface.

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. show system mtu

DETAILED STEPS

  Command or Action Purpose
Step 1

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 2

interface interface

Example:

Device(config)# interface gigabitethernet0/0

Enters interface configuration mode.

Step 3

ip mtu bytes

Example:

Device(config-if)# ip mtu 68

Changes the IPv4 MTU size

Step 4

ipv6 mtu bytes

Example:

Device(config-if)# ipv6 mtu 1280

(Optional) Changes the IPv6 MTU size.

Step 5

end

Example:

Device(config-if)#  end

Returns to privileged EXEC mode.

Step 6

copy running-config startup-config

Example:

Device# copy running-config startup-config

Saves your entries in the configuration file.

Step 7

show system mtu

Example:

Device# show system mtu

Verifies your settings.

Configuration Examples for System MTU

Configuration Examples for System MTU

Example: Configuring Protocol-Specific MTU 

Switch# configure terminal
Switch(config)# interface gigabitethernet 0/0
Switch(config-if)# ip mtu 900 
Switch(config-if)# ipv6 mtu 1286
Switch(config-if)# end

Example: Configuring the System MTU 

Switch# configure terminal
Switch(config)# system mtu 1600
Switch(config)# exit

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.