Setting the MTU for all Layer 3 Interfaces

The global Layer 3 Maximum Transfer Unit (MTU) feature provides a CLI command to set the MTU value for all Layer 3 interfaces.

  • EOS uses a default value for the global Layer 3 MTU and each interface Layer 3 MTU to 1500.
  • The global L3 MTU configuration does not affect an interface configured with an MTU.
  • When changing the global L3 MTU, existing interfaces using the default value 1500 update to use the new MTU. Any additional interfaces configured in the future also use the new default MTU.

This topic contains the following sections:

Configuring MTU for all Layer 3 Interfaces

Use the mtu command in the Interface Configuration Mode. In the following example, set the MTU value to 1600.

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

Displaying MTU Information

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

Example

(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 the previous example, the commands perform the following tasks:
  • Change interface ethernet3 from Layer 2 to Layer 3 using no switchport and explicitly set the MTU to 1600.
  • Enter the Interface Defaults Configuration Mode and set the global L3 MTU to 1700.
  • The show interface command output states that interface ethernet1 and interface ethernet2 use the global L3 MTU value, while interface ethernet3 uses the local MTU value.
  • The default in the output indicates ifinterface uses the global L3 MTU.

Considerations

The following considerations apply to the Global setting forMTU on all Layer 3 interfaces feature:
  • EOS only supports Ethernet, Port-Channel, and VLAN interfaces.
  • Does not support other interface types, such as Management, Loopback, VXLAN, and Tunnel.

Show Commands

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

Example

(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 the previous example, the commands perform the following tasks:
  • Change interface ethernet3 from Layer 2 to Layer 3 using no switchport and explicitly set the MTU to 1600.
  • Enter the Interface Defaults Configuration Mode and set the global L3 MTU to 1700.
  • The show interface command output states that interface ethernet1 and interface ethernet2 use the global L3 MTU value, while interface ethernet3 uses the local MTU value.
  • The default in the output indicates if interface uses the global L3 MTU.