MikroTik RouterOS parameters

The MikroTik RouterOS L2MTU and Max L2MTU parameters are used to configure the Layer 2 Maximum Transmission Unit (MTU) on a network interface.

L2MTU

The L2MTU parameter defines the maximum size of a packet that can be transmitted over a network interface. The default value of L2MTU is 1500 bytes, which is the maximum size of an Ethernet packet.

Max L2MTU

The Max L2MTU parameter defines the maximum size of a packet that can be transmitted over a network interface, regardless of the size of the data link's Layer 2 MTU. The default value of Max L2MTU is 1500 bytes, which is the maximum size of an Ethernet packet.

Difference between L2MTU and Max L2MTU

The difference between L2MTU and Max L2MTU is that L2MTU is based on the size of the data link's Layer 2 MTU, while Max L2MTU is not.

L2MTU and Max L2MTU Settings

To configure L2MTU and Max L2MTU on a MikroTik RouterOS network interface, the mtu command can be used.

/interface ethernet set [interface-name] mtu [l2mtu] [max-l2mtu]

For example, to configure L2MTU on an Ethernet interface with name “ether1” to 1400 bytes and Max L2MTU to 1500 bytes, the following command would be used:

/interface ethernet set ether1 mtu 1400 max-l2mtu 1500

Examples of use

The L2MTU and Max L2MTU parameters can be used to troubleshoot packet fragmentation on a network. If packet fragmentation is occurring, the L2MTU or Max L2MTU value can be increased to prevent packets from being fragmented.

They can also be used to improve the performance of a network. If the data link is known to have a small MTU, the L2MTU or Max L2MTU value can be reduced to avoid wasting bandwidth by sending packets larger than necessary.

MTU at Layer 3 (Network)

At Layer 3 of the OSI model, which refers to the network layer, the MTU is related to the fragmentation of IP packets. Routers and routing devices play a crucial role in managing IP packets and MTU at this layer.

When an IP packet moves from one network to another through a router, the MTU can vary between networks. If an IP packet is larger than the MTU of the destination network, the router must fragment the packet or discard it.

IP packet fragmentation can have significant impacts on network performance and efficiency. Fragmented packets require more processing resources and bandwidth to transmit, which can impact network speed and increase the likelihood of packet loss.

Additionally, some devices and firewalls may block or discard fragmented packets for security reasons, which can lead to connectivity issues.

Considerations for Optimizing the MTU

To ensure optimal performance on a network, it is important to carefully consider the MTU at both layers. Some key considerations include:

1. Knowledge of Technology

Understanding the specific network technology being used and its standard MTU is essential to avoid fragmentation issues.

2. MTU Settings

In some cases, it may be beneficial to adjust the MTU on specific devices to optimize network performance.

3. Path MTU Discovery (PMTUD)

Using mechanisms such as PMTUD to determine the maximum MTU along the communication path can help avoid packet fragmentation.

4. Avoid Fragmentation

Where possible, packet fragmentation should be avoided as it can have a negative impact on network efficiency.

In summary, the MTU at Layer 2 and Layer 3 is a fundamental concept in communication networks. Its proper management is essential to ensure optimal performance and avoid packet fragmentation problems, which can affect network efficiency and reliability.