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Finding Maximum MTU Value

Note:  MDS recommends using a value of 1452 for DSL connections but the below process will tell you the exact max MTU that your upstream network can carry.

 

Prerequisites

1. This test MUST be done while the customer is connected directly to the ISP modem, not the Cloud-Link.

2. It is highly recommended that the MTU value changes are made on the ISP modem itself to reduce the processing load placed on the Cloud-Link.

3. The below commands are for Windows only. If testing from a Mac, open Terminal and substitute the command, "ping -D -s 1480 www.google.com" for the Windows command, "ping www.google.com -f -l 1480".

 

What is MTU?

MTU (maximum transmission unit) of a communications protocol of a layer is the size (in bytes or octets) of the largest protocol data unit that the layer can pass onward. If an MTU packet is too large to be passed, it will be rejected, and then re-transmitted at a smaller size, causing delays in the transmission while the packet is broken down into smaller parts.

 

How to find the Maximum MTU value

To find the maximum MTU value of the ISP's network, open Command Prompt and type the following:

ping www.google.com -f -l 1480

If you receive a message stating, "Packet needs to be fragmented but DF set," Then lower the end value by 10. Continue this process until you receive a good reply to your ping. Once you have received a reply, increase the end value by 1. Increase the end value as necessary until you find the maximum value that allows you to receive a good reply. This value will be your Maximum MTU value.

Once you have the Maximum MTU value, you will then take this value and subtract 20 bits to allow for CAPWAP overhead and another 28 bits for IP/ICMP overhead, as well as other protocols. Log in to the ISP modem GUI, and plug the new number into the MTU section. This will tell the Cloud-Link the maximum transmission size, and allow traffic to flow without being fragmented.

In the screenshot below, you will see that the Maximum MTU value for this particular network is 1472, as transmission at 1473 was fragmented.

 

 

So in the example above, the maximum MTU would be 1472.

 

From the 1472 in the above test, subtract 48 for Cloud-Link tunnel overhead you would be at 1424.  In this case you would set MTU value on your modem to 1424.

 

Note:  MDS recommends using a value of 1424 but the above process will tell you the exact max MTU that your upstream network can carry.

 

 

I have DSL and I want to set MTU to 1452, now what?

 

Please contact the locations ISP support.  This is going to be the fastest way to get you configured properly.  Each vendors DSL modem is different.  If you have access to the modem, look for WAN specific connection information and look for the MTU settings within the options you have access to.  MDS strongly suggests working with the ISP to make sure it is changed in the right place.

Special note:  Some ISP provided DSL modems DO NOT have the option to set MTU.  In this case, MDS suggests asking the DSL provider what other DSL modems are compatible and if they could replace the current one with one that has the MTU setting.  Some partners have found replacing with a compatible 3rd party modem that is capable of setting MTU solved this limitation.

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