Configure System MTU

Feature History for MTU

This table provides release and platform support information for the features explained in this module.

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

Table 1.

Release

Feature Name and Description

Supported Platform

Cisco IOS XE 17.18.1

The Maximum Transmission Unit (MTU) features discusses about two primary types of MTU:

  1. System MTU (Global MTU)

  2. Per-Port MTU (Interface MTU)

Cisco C9350 Series Smart Switches

Cisco C9610 Series Smart Switches

Maximum Transmission Unit

The Maximum Transmission Unit (MTU) in network switching refers to the largest size of a packet or frame, measured in bytes, that a network interface can send or receive. If a packet exceeds the MTU of an interface, the system must either fragment the packet into smaller pieces before transmission or drop the packet.

In Cisco switching, you typically configure two primary types of MTU:

  1. System MTU (Global MTU)

  2. Port MTU (Interface MTU)

Configure System MTU

Modify the Maximum Transmission Unit (MTU) settings for optimized network performance. MTU defines the largest size of packets that can be transmitted over the network without fragmentation. Configuring an appropriate MTU size can improve transmission efficiency and reduce latency.

Before you begin

Ensure you have the necessary privileges to access the device in EXEC mode.

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

Displays MTU byte information

Step 4

end

Example:

Device(config)# end

Enters global configuration mode, and 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 platform so fed switch active ifm interface_name interface name

Example:

Device# show platform software fed switch active ifm interface_name GigabitEthernet2/0/1
Interface Name: GigabitEthernet2/0/1
if_id: 0x123
admin_state: up
oper_state: up
l2_mtu: 9198
ip_mtu: 1500
encap: ethernet
vlan_mode: access
access_vlan: 10
l2_fwd: enabled
l3_fwd: disabled
asic_port: 25

Displays the IP/IPv6 MTU.

The MTU size for switched packets is configured successfully, and changes are confirmed to persist upon a system restart.

Configure 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 832

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

Exits interface configuration mode, and 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
System MTU size is 9198 bytes

Verifies your settings.

Configuration examples for System MTU

Protocol-specific MTU configuration example

Protocol-specific MTU configurations allow network professionals to adjust the maximum transmission unit (MTU) for different protocols to optimize performance and minimize packet fragmentation. This is crucial for efficiently managing network traffic across various interfaces.

This example shows how you can configure protocol-specific MTU:

  • Gigabit Ethernet Interface

    Device# configure terminal 
Device(config)# interface gigabitethernet 0/1 
Device(config-if)# ip mtu 900 
Device(config-if)# ipv6 mtu 1286 
Device(config-if)# end

  • Forty Gigabit Ethernet Interface

    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

Effective MTU configuration reduces the risk of fragmentation and ensures efficient packet transmission across network segments.

System MTU configuration example

Change the maximum transmission unit (MTU) size for a Cisco network device. Context: This task is useful for adjusting the packet size to accommodate specific network performance requirements. This example shows how you can configure 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 (Cisco 9350 Series Smart Switches)

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 (Cisco 9610 Series Smart Switches)

Standards and RFCs

Standard/RFC Title
RFC 8200 Internet Protocol, Version 6 (IPv6) Specification