Software Configuration Guide, Cisco IOS XE Denali 16.2.x (Catalyst 3650 Switches)

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Updated:: March 30, 2016

Chapter: Configuring System MTU

Finding Feature Information
Information about the MTU
- Restrictions for System MTU
- System MTU Value Application
How to Configure MTU Sizes
- Configuring the System MTU
- Configuring Protocol-Specific MTU
Configuration Examples for System MTU
Additional References for System MTU
Feature Information for System MTU

Configuring System MTU

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 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 device interfaces is 1500 bytes.

Restrictions for System MTU
System MTU Value Application

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 does not take effect on the device. This command only affects the system MTU size on Fast Ethernet switch ports.

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:

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 or the system jumbo 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

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: 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`	(Optional) 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	reload Example: Device# `reload`	Reloads the operating system.
Step 8	show system mtu Example: Device# `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:

Device(config)# system mtu 7500system mtu 1900
Device(config)# 
Device(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:

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

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

Device# 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

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

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