How to set MTU size

WHAT TO EXPECT

In this article, users will learn how to change the MTU size of a given VM.

In some cases, the MTU Size of the multicast group may exceed the default limit of the OS. It can also happen that, in some circumstances, the user may need a small MTU size to send smaller packages. This article will explain how to increase the MTU size if this should occur.

To know if the MTU size has been exceeded, Wireshark or tcpdump can be used.  Below is an example from Wireshark.

NOTE

In this example, the UDP length error shows it is exceeding the Length.

Change MTU size on Linux

1. First, check the current MTU size by running the following command:

   ifconfig | grep mtu

   Example:

   $ ifconfig | grep mtu
   enP43852s1: flags=6211<UP,BROADCAST,RUNNING,SLAVE,MULTICAST>  mtu 1500
   enP4589s2: flags=6211<UP,BROADCAST,RUNNING,SLAVE,MULTICAST>  mtu 1500
   eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
   eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
   lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536

   As can be seen, the MTU size of eth1 is 1500.

2. To change the MTU size to, for example, 2000, use the command below:

   sudo ifconfig eth1 mtu 2000 up

3. Validate the change is set by running again:

   ifconfig | grep mtu

   Example:

   $ ifconfig | grep mtu
   enP43852s1: flags=6211<UP,BROADCAST,RUNNING,SLAVE,MULTICAST>  mtu 1500
   enP4589s2: flags=6211<UP,BROADCAST,RUNNING,SLAVE,MULTICAST>  mtu 2000
   eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
   eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 2000
   lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536

   As can be seen, the MTU size of eth1 is now 2000.

Change MTU size on Windows

1. Check the current MTU size by running:

   netsh interface ipv4 show subinterfaces

   That will show a list of network interfaces with their MTU size:

   PS C:\Windows\System32> netsh interface ipv4 show subinterfaces
   
          MTU  MediaSenseState      Bytes In     Bytes Out  Interface
   ----------  ---------------  ------------  ------------  -------------
   4294967295                1             0        444816  Loopback Pseudo-Interface 1
         1500                5             0          3214  Local Area Connection* 1
         1500                5             0             0  Local Area Connection* 2

   As can be seen, the MTU size of the local adapters is 1500.

2. Set the MTU Size (in this case, to 2000) using the following commands:

   netsh interface ipv4 set subinterface "Interface Name" mtu=2000 store=persistent

   Where “Interface Name” is the Ethernet adaptor to be set. For example, users can get the list of names with ipconfig:
   

   1. If the following error appears:

      PS C:\Windows\System32> netsh interface ipv4>set subinterface "Ethernet 2" mtu=2000 store=persistent
      The parameter is incorrect.

      Then follow these steps:

      1. Go to the Network connection → “Control Panel\Network and Internet\Network Connections”
         

      2. Right-click on the Ethernet Adaptor that the traffic is expected and select Properties.
         

      3. Select “Configure”
         

      4. Go to Advanced, look for and select "Jumbo Packet":
         

      5. Select the desired size, for example"4088 Bytes", then select "OK".
         

      6. Re-run step 2 to set MTU size.

      7. If needed, reboot the VM.

      8. Re-run step 1 to validate the MTU size is correct