Configure Per-Port MTU

Restrictions for configuring per-port MTU

Several restrictions apply to configuring per-port MTU on Cisco Catalyst switches. These restrictions vary depending on the platform, interface type, and other configuration conditions.

General restrictions:

  • Per-port MTU cannot be configured on the management port.

  • Do not configure the per-port MTU value while traffic is flowing (e.g., during active data transmission).

Port-channel restrictions:

  • Members of a port channel cannot be configured with a per-port MTU value; they derive their MTU from the port-channel MTU configuration.

  • Per-port MTU can be configured on port-channel interfaces but not on the port-channel members.

Subinterface restrictions:

  • Per-port MTU is not supported on subinterfaces or port-channel subinterfaces.

Layer 2 MTU restrictions:

  • Ingress and egress Layer 2 MTU is derived from per-port MTU. If per-port MTU is not configured, the System MTU is used.

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

Platform-specific restrictions:

  • On Cisco Catalyst 9610 Series Supervisor 2 Module, the following restrictions are applicable:

    • If no protocol-specific MTU configuration is present, per-port MTU is used as protocol-specific MTU.

    • If per-port MTU is not configured, System MTU is used as protocol-specific MTU.

Port MTU specifications for Cisco Catalyst switches

You can configure the MTU size for all interfaces on a device at the same time using the system mtu command. The default maximum transmission unit (MTU) size for frames received and transmitted on all interfaces is 1500 bytes. The system mtu command is a global command and does not allow MTU to be configured at a port level. Starting with Cisco IOS XE 17.1.1, you can configure Per-Port MTU. Per-Port MTU will support port level and port channel level MTU configuration. With Per-Port MTU you can set different MTU values for different interfaces as well as different port channel interfaces.

Per-port MTU can be configured in the range of 1500-9198 bytes.

Per-port MTU can be configured in the range of 1500-9216 bytes.

Once the Per-Port MTU value has been configured on a port, the protocol-specific MTU for that port is also changed to the Per-Port MTU value. When Per-Port MTU is configured on a port, you can still configure protocol-specific MTU on the interface in the range from 256 to Per-Port MTU value.

If the Per-Port MTU is disabled, the MTU for the port will revert to the system MTU value.

You can view the Per-Port MTU configurations on an interface using the show interface mtu command.

The following are expected behaviour if the Per-Port MTU configuration is changed on any interface:

  • The interface flaps if the port-channel is in PAgP or LACP mode.

  • The interface does not flap if the port channel is in the on mode.

  • The interface does not flap if the interface is not a port channel.

You can disable Per-Port MTU by using the no form of the mtu bytes command in the interface configuration mode.

NOTE:

To form routing protocol adjacency, ensure port MTUs match between devices. Configure matching IP MTU or IPv6 MTU values on the interfaces of both devices.

Configure per-port MTU

Follow these steps to configure per-port MTU on an interface:

Procedure

Command or Action Purpose

mtu port

Example:

Device# configure terminal
Device(config)# interface FortyGigabitEthernet2/5/0/20
Device(config-if)# mtu 6666 
Device(config-if)# end

Configuration Examples for Per-Port MTU

The following displays an example for Per-Port MTU configuration on an interface:

Device# configure terminal
Device(config)# interface FortyGigabitEthernet2/5/0/20
Device(config-if)# mtu 6666 
Device(config-if)# end

Verify per-port MTU

The following example illustrates how to verify the per-port MTU configuration on an interface using the show interface mtu command.

Procedure

Command or Action Purpose

show interface mtu

Example:

Device# show interface mtu
Port           Name               MTU         
Fo2/5/0/19                        1500        
Fo2/5/0/20                        6666        
Fo2/5/0/21      ixia_7_21         1500

Disable per-port MTU

This example shows how to disable Per-Port MTU on an interface:

Before you begin

  • Ensure you have administrative access to the device.

  • Confirm the interface ID where you want to disable the per-port MTU.

Procedure

Command or Action Purpose

no mtu

Example:

Device# configure terminal
Device(config)# interface FortyGigabitEthernet2/5/0/20
Device(config-if)# no mtu 
Device(config-if)# end