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All about MTU

warning

Expert function​

Swisscom recommends that this function should only be configured by experienced IT specialists. Misconfigurations can lead to considerable security risks, which can negatively affect both the customer installation and Swisscom's reputation.

**Details can be found in the FAQ - MTU.


The Maximum Transmission Unit (MTU) is an important term in network technology that describes the largest possible packet size that can be transmitted via a network protocol. It is measured in bytes and directly influences the efficiency of data transmission in networks.

Goal and purpose​

A correct MTU setting is crucial for network performance. If the MTU is set too high, fragmentation can occur, i.e. large packets are broken down into smaller parts, which can impair the efficiency of the transmission. However, if the MTU is too low, the available bandwidth is not optimally utilised as more packets have to be sent to transfer the same amount of data.


MTU settings for different network areas​

Here are the maximum MTU settings for different network areas and versions of the router CB2.0 and CB3.0.


Valid for: FTTH, XGSPon, xDSL​

Network areaCB2.0 MTUCB3.0 MTU
LAN15001500
DMZ15001500
IP Passthrough15001500
PPP Passthrough15081508
1:1 NAT15001500

Valid for: 5G & 4G Mobile Access​

Network areaCB2.0 MTUCB3.0 MTU
LAN14341434
DMZ14341434
IP Passthrough14341434
1:1 NAT14341434

These settings are crucial for the optimum configuration and performance of the network in the areas mentioned.

FAQ - MTU​

Why is the correct MTU setting important?

The correct MTU setting is crucial for optimising network performance and efficiency. An MTU that is too low can lead to an increased number of packets, while an MTU that is too high can lead to fragmentation, both of which can impair network performance.

How can I check the MTU of my connection?

The MTU of the connection can be checked and customised on various operating systems. Here are the steps for some common systems:

  • Windows:

    • Open the command prompt by pressing the Windows key, entering "cmd" and pressing Enter.
    • Use the following command to display the MTU for your network adapters: 
      netsh interface ipv4 show subinterfaces
    • In the output text, you can see the "MTU" column, which shows you the currently set MTU for each network adapter.

  • Linux/Unix:

    • Open a terminal.
    • Use ip or ifconfig to display the MTU details. The ip command is more modern and favoured: 
      ip link show
    • Search for "mtu" in the output, which is displayed next to each network interface.
    • Alternatively, you can also use ifconfig, although it may not be installed by default on newer systems: 
      ifconfig
    • The MTU is displayed in the section for each interface.

  • macOS:

    • Open the terminal.
    • Use ifconfig to check the MTU settings: 
      ifconfig
    • Here too, you will find the MTU values directly in the interface details.

These commands show you the currently configured MTU for each network interface and help you to gain information on how to adjust the MTU if necessary.

How do I determine the optimum MTU?

To determine the optimum MTU, ping commands are used to test different packet sizes step by step until the maximum size is found that can be transmitted without fragmentation.

  • Windows:

    • Open the command prompt by pressing the Windows key, entering "cmd" and pressing Enter.
    • Use the following command to determine the MTU:
    ping <zielhost> -f -l <paketgröße>

  • -f prevents fragmentation

  • -l specifies the packet size (MTU = packet size + 28 byte header)

  • Linux:

    • Open a terminal.
    • Use the following command to determine the MTU:
    ping <zielhost> -M do -s <paketgröße>

  • -M do means "Don't Fragment" - the package must not be fragmented.

  • -s is the size of the payload (MTU = payload + 28 byte header).

  • macOS:

    • Open a terminal.
    • Use the following command to determine the MTU:
    ping -D -s <paketgröße> <zielhost>

  • -M do means "Don't Fragment" - the package must not be fragmented.

  • -s is the size of the payload (MTU = payload + 28 byte header).

The MTU specifies the maximum possible size of a data packet in a network and includes the user data and the protocol header. When transmitting via IP networks, a data packet typically consists of the IP header (usually 20 bytes) and the ICMP header (8 bytes), so that of a standard MTU of 1500 bytes, 28 bytes are effectively reserved for these headers and 1472 bytes are available for the actual user data.

What happens if a package is larger than the MTU?

If a packet exceeds the MTU, it must be fragmented in order to be transmitted. Fragmentation breaks up large packets into smaller pieces, which can put more strain on network resources and potentially slow down the entire transmission.

What is the optimum MTU size?

The optimum MTU size depends on the respective network and may vary. For many Internet connections, an MTU size of 1500 bytes is standard, although smaller or larger values may be preferred in certain network scenarios.

Can an incorrect MTU setting lead to network problems?

Yes, an incorrect MTU setting can lead to network problems such as packet loss, increased latency and inefficient bandwidth utilisation. It is important to define and test a suitable MTU for the respective network.

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