Configuring System MTU

Restrictions for System MTU

On Cisco Catalyst 9500X Series switches, the following restrictions are applicable:

  • If no protocol-specific MTU configuration is present, Per-Port MTU is used as protocol-specific MTU. In case Per-Port MTU is not configured, System MTU is used as protocol-specific MTU.

  • Ingress and egress Layer 2 MTU is derived from Per-Port MTU. If Per-Port MTU is not configured, System MTU is used

  • On ingress ports configured with Layer 2 MTU, if packets exceed the configured MTU size, then the packets are dropped.

  • Layer 2 MTU configurations are not enforced for egress frames.

Information About the MTU

The default maximum transmission unit (MTU) size for payload received in Ethernet frame and sent on all device interfaces is 1500 bytes.The Cisco Catalyst 9500 Series Switches support a range from 1500 to 9198 bytes. The Cisco Catalyst 9500 Series Switches - High Performance and Cisco Catalyst 9500X Series Switches support a range of 1500 to 9216 bytes.

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

The Cisco Catalyst 9500 Series Switches support a range of 1500 to 9198 bytes. The Cisco Catalyst 9500 Series Switches - High Performance and Cisco Catalyst 9500X Series Switches support a range of 1500 to 9216 bytes.

Use the ip mtu bytes command.

The range is from 832 up to 1500 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 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.

Beginning from Cisco IOS XE Amsterdam 17.3.x, the minimum IPv6 system MTU is fixed at 1280 as per RFC 8200.

How to Configure MTU

Configuring the System MTU

Follow these steps to change the MTU size for switched packets:

Procedure

  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 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

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. To change the MTU size for routed ports, perform this procedure

Procedure

  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

Example: Configuring Protocol-Specific MTU

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

Example: Configuring the System MTU 

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

Additional References for System MTU

Related Documents

Related Topic Document Title

For complete syntax and usage information for the commands used in this chapter.

See the Interface and Hardware Commands section in the Command Reference (Catalyst 9500 Series Switches)

Standards and RFCs

Standard/RFC Title
RFC 8200

Internet Protocol, Version 6 (IPv6) Specification

Feature History for System MTU

This table provides release and related information for features explained in this module.

These features are available on all releases subsequent to the one they were introduced in, unless noted otherwise.

Release

Feature

Feature Information

Cisco IOS XE Everest 16.5.1a

System MTU

System MTU defines the maximum transmission unit size for frames transmitted on all interfaces of a switch.

Support for this feature was introduced on the C9500-12Q, C9500-16X, C9500-24Q, C9500-40X models of the Cisco Catalyst 9500 Series Switches.

Cisco IOS XE Fuji 16.8.1a

System MTU

System MTU defines the maximum transmission unit size for frames transmitted on all interfaces of a switch.

Support for this feature was introduced on the C9500-32C, C9500-32QC, C9500-48Y4C, and C9500-24Y4C models of the Cisco Catalyst 9500 Series Switches.

Use Cisco Feature Navigator to find information about platform and software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn.