mtu

Use this command to set the Maximum Transmission Unit (MTU) and Maximum Receive Unit (MRU) for an interface

Use the no parameter with this command to set the MTU to its default.

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To allow jumbo frames over SVI interfaces, it is mandatory to configure the applicable MTU for the specific SVI interfaces.

Limitation for MTU configuration on Label-Switching

Creating a sub-interface automatically increases the physical interface MTU size by 8 bytes to accommodate double VLAN tag encapsulation.

Configuring label switching for physical layer-3 interfaces adds 20 bytes internally to the MTU to accommodate up-to five labels. However, configuring label-switching on sub-interface does not change the MTU of physical interface. Hence, the physical interface requires a manual increase in MTU size.

During the BGP update, in case the control packet contains 1500 bytes when it reaches the hardware, the hardware adds the Encapsulation for the sub-interface and MPLS header (Additional bytes). Now, the hardware drops it as physical port MTU is limited to 1500 bytes.

While configuring MTU on label-switching enabled with Subinterface/SVI/LAG and the Parent Physical port follow guide lines mentioned below:

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It is recommended to configure higher MTU on network ports in comparison with access ports. Hence, increase the MTU on both physical and sub-interfaces to accommodate the PDU.

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When using sub-interface for MPLS network interfaces, considering the default MTU of 1500, minimum MTU configuration recommendation is as follows

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Sub-interface: MTU 1520 (to accommodate 5 MPLS labels)

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Physical interface: MTU 1528: (Default MTU 1500 + double encap 8 + MPLS up-to 5 labels 20) = 1528).

MTU configuration is considered from IP header onwards. Hence, OcNOS adds 14 bytes to MTU internally to accommodate L2 header. The effective MTU in hardware will be 1528+14 = 1542.

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LAG interface: MTU is applied on all members internally

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SVI: When label-switching enabled on VLAN interface, MTU value must be manually increased by at least 20 bytes on Parent interfaces of VLAN.
Example, default MTU must be set as 1520 instead of 1500 on label-switching parent interface label switched VLAN interface. (Parent Interface MTU >= label switched VLAN interface MTU + 20).

Command Syntax

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mtu <64-65536>
no mtu

Parameters

<64-65536>

Specify the size of MTU in bytes:
<64-16338> for L2 packet
<576-9216> for L3 IPv4 packet
<1280-9216> for L3 IPv6 packet
<576-65536> for IPv4 packet
<1280-65536> for IPv6 packet on loopback interface

Default

By default, MTU is 1500 bytes

Command Mode

Interface mode

Applicability

This command was introduced before OcNOS version 1.3.

Example

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#configure terminal
(config)#interface eth3
(config-if)#mtu 120