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Everything about MTU

warning

Expert Function​

Swisscom recommends that this function only be configured by experienced IT specialists. Misconfigurations can lead to significant security risks, which may negatively impact both the customer installation and Swisscom’s reputation.

Details can be found in the FAQ - MTU.


The Maximum Transmission Unit (MTU) is an important concept in network engineering that defines the maximum packet size that can be transmitted over a network protocol. It is measured in bytes and directly influences the efficiency of data transmission in networks.

Purpose and Objective​

A correct MTU configuration is crucial for network performance.
If the MTU is set too high, fragmentation may occur — meaning large packets are split into smaller pieces, which can reduce transmission efficiency.
If the MTU is set too low, the available bandwidth is not used efficiently, as more packets need to be sent to transmit 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 CB2.0 and CB3.0 routers.


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 essential for the optimal configuration and performance of the network in the areas mentioned.

FAQ - MTU​

Why is the correct MTU setting important?

The correct MTU configuration is essential for optimising network performance and efficiency.
A MTU that is too low can lead to an increased number of packets, while an MTU that is too high can cause fragmentation — both of which can negatively impact network performance.

How can I check the MTU of my connection?

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

  • Windows:

    • Open the Command Prompt by pressing the Windows key, typing “cmd”, and pressing Enter.
    • Use the following command to display the MTU values of your network adapters: 
      netsh interface ipv4 show subinterfaces
    • In the output, look for the “MTU” column showing the currently configured MTU for each network adapter.

  • Linux/Unix:

    • Open a terminal.
    • Use ip or ifconfig to display MTU information. The ip command is more modern and preferred: 
      ip link show
    • Look for “mtu” next to each network interface.
    • Alternatively, you can use ifconfig (may not be installed by default on modern systems): 
      ifconfig

  • macOS:

    • Open the Terminal.
    • Use the following command to check MTU settings: 
      ifconfig
    • The MTU values appear directly within the interface details.

These commands show the MTU currently configured for each interface and help you determine whether an adjustment may be required.

How do I determine the optimal MTU?

To determine the optimal MTU, different packet sizes are gradually tested using ping commands until the largest value that can be transmitted without fragmentation is found.

  • Windows:

    ping <targethost> -f -l <packetsize>
    • -f prevents fragmentation
    • -l sets the packet size (MTU = packet size + 28 bytes header)

  • Linux:

    ping <targethost> -M do -s <packetsize>
    • -M do means “Don't Fragment”
    • -s defines the payload size (MTU = payload + 28 bytes header)

  • macOS:

    ping -D -s <packetsize> <targethost>

A MTU represents the maximum possible size of a data packet in a network and includes both the payload and the protocol header.
In IP networks, a typical packet consists of an IP header (usually 20 bytes) and an ICMP header (8 bytes),
so from a standard MTU of 1500 bytes, 28 bytes are reserved for headers, leaving 1472 bytes for actual payload.

What happens if a packet is larger than the MTU?

If a packet exceeds the MTU, it must be fragmented before it can be transmitted.
Fragmentation splits large packets into smaller parts, increasing resource usage and potentially slowing down overall transmission.

What MTU size is optimal?

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

Can an incorrect MTU setting cause network issues?

Yes, an incorrect MTU setting can lead to packet loss, increased latency, and inefficient bandwidth usage.
It is important to choose and test an MTU that fits the requirements of your network environment.

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