Maximum Transmission Unit (MTU) is the largest Protocol Data Unit (PDU) that can be sent in a single network layer transaction

The maximum MTU size for an Ethernet frame is 1500 bytes [Max Ethernet frame size 1518 bytes -18 bytes for overhead (header and frame check sequence)]

Larger MTU sizes are more efficient as it allows more data to be transmitted in each data packet. However overhead in Internet connections will require the MTU size to be reduced to cater for the overhead. For example, PPPoE header uses 8 bytes and a VLAN tag uses 22 bytes. So, for an Internet connection which uses PPPoE, the MTU size will be 1492.

What happens if large MTU value is used

IPv4 allows fragmentation which divides the datagram into pieces, each small enough to accommodate a specified MTU limitation. However, this can cause unnecessary overhead slowing down communication.

For VPN a large MTU size can cause data loss resulting in failure to establish the VPN connection.

There are several methods to determine the optimum MTU size to use for router Internet connections that will be covered below:

Method 1: Ping Test

This method uses the ping utility in the PC to check for a suitable MTU size.

In a Windows PC, go to the search bar and enter cmd.

This will bring up the DOS prompt.

Now run the ping command to the site you wish to connect to.

Ping remote-host -f -l size

Where

-l size      Send buffer size.

-f             Set Don’t Fragment flag in packet (IPv4-only).

 

You can start with a packet size of 1500 and lower the size in increments until you do not see fragmentation.

For example, testing to 8.8.8.8 you will see the following results:

Ping 8.8.8.8 -f -l 1500

The result shows that the data packet needs to be fragmented.

Repeat this test by using a lower packet size until you do not see the fragmentation message.

From our test we find that the maximum data packet size before fragmentation is 1472 bytes.

For example, if the largest packet size from ping tests is 1472, add 28 to 1472 to get a total of 1500 which is the optimal MTU setting. (If ping test gives 1460, adding 28 gives you MTU size of 1488)

Change the MTU on the router WAN Setup page.

Method 2: Path MTU Discovery Tool

DrayTek routers running the latest firmware have the Path MTU Discovery tool that can be used to find the best MTU size to reach a destination.

This is available in WAN >> Internet Access menu in the details tab.

The default MTU size of 1500 bytes may be too large to connect to certain sites via VPN. For example, NordVPN may require a smaller MTU size, usually 1492 or smaller.

To check the maximum MTU size that can be used to establish a NordVPN connection, go to WAN >> Internet Access menu and click on “Path MTU Discovery” button.

  • Enter IP address of the Nord VPN server you are going to connect to
  • Click on “Detect” and the router will report a suitable MTU size to use

Method 3: Using Online Tools

There are some online tools that can be used to check for the maximum MTU size for a destination on the Internet.  One tool that can be used is “MTU test”  http://www.letmecheck.it/mtu-test.php

This utility uses ping test to find the maximum MTU size, similar to what was described in method 1.

Enter the host IP address and run the utility.

When completed, it will provide the results similar to that shown below:

Sending 1454 bytes to 104.200.132.226  <-  not fragmented

Sending 1478 bytes to 104.200.132.226  <-  FRAGMENTED!

Sending 1466 bytes to 104.200.132.226  <-  not fragmented

Sending 1472 bytes to 104.200.132.226  <-  not fragmented

Sending 1475 bytes to 104.200.132.226  <-  FRAGMENTED!

Sending 1473 bytes to 104.200.132.226  <-  FRAGMENTED!

Sending 1472 bytes to 104.200.132.226  <-  not fragmented

 

From the tests we did, we can assume that 1472 bytes is the largest unfragmented packet

size. The MTU size would be 1500, made up from 1472 payload and 28 ICMP/IP Headers

and payload information.

The maximum MTU size for 104.200.132.226 is:    1500