Checking the WAN Connection from the Carrier - how-to-determine-the-mtu-size-of-the-lan-and-wan-of-your-router/Checking-the-WAN-Connection-from-the-Carrier

Determining Correct Maximum Transmission Unit Size
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en-US
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Paligo_Prod
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Article

To ping the default gateway, connect to your router, log in to its admin pages, and go to Status > Internet > Connections. Check the box corresponding to your WAN source and get the IP address for your default gateway.

Note

Some carriers block WAN pings, so you may not be able to ping the gateway or DNS servers of your network. 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 testing this, start with the values suggested above. If you get replies, increase the Maximum Transmission Unit (MTU) value.

When you receive “Packet needs to be fragmented but DF set” rather than replies, your MTU is too high. Lower it and you can get the MTU set to the absolute ceiling (where +/- 1 will reply / fragment-out).