Global Knob to Set MTU for all Layer 3 Interfaces

  • Global Layer 3 Maximum Transfer Unit (MTU) feature provides a CLI command to set the MTU value for all Layer 3 interfaces.
  • The default value for global Layer 3 MTU and for each interface Layer 3 MTU is 1500.
  • Any interface which has an MTU configured is not affected by the global L3 MTU.
  • When the global L3 MTU is changed, any existing interfaces which are using the default are updated to use the new MTU. Any additional interfaces which are configured in the future will also use the new default MTU.

Therelated sections are:

Global Knob to Set MTU for all Layer 3 Interfaces Configuration

The global L3 mtu command is under interface-defaults sub-mode. In this example, the MTU value is set to 1600.

(config)# interface defaults
(config-interface-defaults)# mtu 1600

Limitations

The following are limitations for the Global knob to set MTU for all Layer 3 interfaces feature:
  • Only Ethernet, Port-Channel, and VLAN interfaces are supported.
  • Other interface types, such as Management, Loopback, VXLAN, and Tunnel, are not supported.

Show Commands

Use the show interface command to display the current MTU value applied on the selected interface. In the following example,interface ethernet1, interface ethernet2, and interface ethernet3 are selected for display.

(config)# interface ethernet3
(config-if-Et3)# no switchport
(config-if-Et3)# mtu 1600
(config-if-Et3)# interface defaults
(config-interface-defaults)# mtu 1700
(config-interface-defaults)# show interfaces ethernet1
  ...
  IP MTU 1700 bytes (default) , BW 10000000 kbit   
  ...
(config-interface-defaults)# show interfaces ethernet2
  ...
  IP MTU 1700 bytes (default) , BW 10000000 kbit
  ...
(config-interface-defaults)# show interfaces ethernet3
  ...
  IP MTU 1600 bytes , BW 10000000 kbit
  ...

In this example,
  • Switch interface ethernet3 from Layer 2 to Layer 3 and explicitly set its MTU to 1600.
  • Enter the interface-defaults sub-mode, set global L3 MTU to 1700.
  • The show interface command states interface ethernet1 and interface ethernet2 are using global L3 MTU value, while interface ethernet3 are using its local MTU value.
  • The default in the output indicates whether or not the interface is using global L3 MTU.