Determine MTU Ceiling Size of Routes LAN and WAN

Jesse Rothschild

Is there a way to determine the MTU size of the LAN and WAN of my router?

We can use the command line to ping with specific MTU sizes, and set a flag to tell us when we fragment-out (meaning the MTU at this point it too high). In doing this, we are able to find the ceiling of your LAN connection, as well as the WAN packets coming from your carrier. It’s a bit tedious, but it works.


Here are the MTU ceiling sizes you should be around:

For the LAN connection to your computer: 1500

If you are connected to WiMAX (WAN packets from your carrier): 1400

If you are connected to 2G/3G, (WAN packets from your carrier): should be anywhere from 1350 – 1500 (it really does vary this much – it’s carrier dependent).

If you are connected to LTE: Modem WAN should be set to 1428.

If you are using a Cable Modem WAN connection: 1500

If you are using a DSL WAN connection: 1492


Configuration

Configuration Difficulty: Intermediate

Getting to the Command Line:

In Win XP:

  • Start -> Run -> type in: cmd -> hit Enter

In Win Vista, Win 7, or Win 8:

  • Start -> in the search bar type in: cmd -> hit Enter

On a Mac OS:

  • Go to /Applications/Utilities/Terminal.app

Sample layout of the command:

ping –f –l (this is a lowercase “L”) [MTU size] [destination]

How to check your LAN connection of your router to your computer:

ping –f –l 1500 192.168.0.1

How to check your WAN connection from your carrier:

Connect to your router and login to its admin pages, go to STATUS > Internet > Connections > check the box corresponding to your WAN source, get the IP address for your Default Gateway – we’ll want to ping this but some carriers block WAN pings, so we may not be able to ping the Gateway or DNS Servers of your network, but if you can, try them first. If you cannot, ping “google.com” instead, or any other website that doesn’t block WAN pings.

ping –f –l 1400 [Default Gateway]

or

ping –f –l 1400 google.com

When you are testing this, start with the values suggested above, if you get replies, great, increase the MTU value. When you get “Packet needs to be fragmented but DF set.” Rather than replies, your MTU is too high now, so lower it and you can get the MTU set to the absolute ceiling (where +/- 1 will reply / fragment-out).

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