How To Find Best MTU Value For Your Internet Connection

MTU packet size that comes by default in any Windows installation may not be the best size. We can make it the best size.

Open a Command Prompt window with admin rights and type in the following command to see what you have:

netsh interface ipv4 show subinterfaces

You will get a MTU value for your internet connection. This is Ethernet in my case and its value is 1500 (see below screenshot).



I will test if this is a good value. Now we should subtract 28 from this value (1500 - 28 = 1472) and test if we get fragmented packets.

(28 is the header of packets. We subtract it to get the actual packet size)

In the Command Prompt window, type in:

ping www.tp-link.com -f -l 1472

(I used tp-link internet address. You can use any internet address, such as www.google.com)



As you can see, I get packets fragmented. I lost 3 packets out of 4 sent during the ping test. So MTU=1500 is not a good value for my system.

Now I decrease this value by 10 or 12 -doesn't matter- (1472 -12 = 1460). Test again.

This value 1460 came clean. No fragmented packets. But we should continue to find the best value.

If you don't get fragmentation, you shoud increase the value 1460 by a small amount (let's say 2) and test again.

I tested 1462 and 1464. They came clean.



Then I tested 1466. Then fragmented packets started again.



So I used 1464.

Now we must add 28 (Packet header) to 1464. 1464 + 28 = 1492. Our MTU size is 1492.

We must set this value in our computer.

Again in Command Prompt window with admin rights, type in, one by one:

netsh

interface

ipv4

The prompt should have been changed to the following: netsh interface ipv4>

Now type in:

set subinterface Ethernet mtu=1492 store=persistent

If your Ethernet has a different name, like Ethernet 2 or Ethernet 3 or another name, use that name directly in the above command instead of Ethernet.

Now we can check (in Command Prompt window with admin rights) if we want. Type in:

netsh interface ipv4 show subinterfaces



As you can see, I have changed MTU size and made it remain (persistent) in my system.

Hope you find this post useful.

my MTU is set to 4088 jumbo frame for the ethernet connection and no dropped packets.
i am running a 1GB fibre pipe.

best of luck, Steve ..

XxXxX said:

my MTU is set to 4088 jumbo frame for the ethernet connection and no dropped packets.
i am running a 1GB fibre pipe.

best of luck, Steve ..

Click to expand...

You're flying at high altitude and at very high speed. I am jealous of you.

I have Verizon FiOS gigabit, and I don't see any errors with the default 1500 setting.

Pinging www.tp-link.com [13.224.214.46] with 1472 bytes of data:
Reply from 13.224.214.46: bytes=1472 time=4ms TTL=247
Reply from 13.224.214.46: bytes=1472 time=4ms TTL=247
Reply from 13.224.214.46: bytes=1472 time=3ms TTL=247
Reply from 13.224.214.46: bytes=1472 time=4ms TTL=247

Ping statistics for 13.224.214.46:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 3ms, Maximum = 4ms, Average = 3ms

I remember when setting the correct MTU was a thing many years ago. If I'm remembering correctly this was more of a concern with DSL connections but not as much of an issue with Cable/Coax connections. Eventually I stopped worrying and making any changes but forget now why. Something about Windows getting better at detecting and setting the proper value on it's own. For a long period of time I used a Utility called "TCPOptimizer" to automatically set many. Got it from SpeedGuide.net.

I remember those programs, but since getting gigabit connections, things happen more than fast enough for me and I haven't really thought about it.

BamaInArk said:

I remember when setting the correct MTU was a thing many years ago. If I'm remembering correctly this was more of a concern with DSL connections but not as much of an issue with Cable/Coax connections. Eventually I stopped worrying and making any changes but forget now why. Something about Windows getting better at detecting and setting the proper value on it's own. For a long period of time I used a Utility called "TCPOptimizer" to automatically set many. Got it from SpeedGuide.net.

Click to expand...

Different network media types used by your ISP have different MTU values, but as most providers are using fiber internally (except for the "last mile" to your house if you're not Verizon or Google Fiber), it's become less of a problem.

If you're using a 5G mobile hotspot or satellite data, it can still be a concern.

@gunrunnerjohn

If your system does not display fragmented packets at default MTU=1500, then you are good to go. No adjustments required. You're among the lucky guys in the world.

Microsoft's default data of MTU=1500 must be good for many systems. There must be a reason for that select data. But for some systems, like mine, it may be out of order.

Happy computing.

Ethernet's normal frame size allows a payload of 1500 bytes, or the default MTU that Windows chooses. Your ISP's network equipment may use a different transmission technology that drops their frame size down, or bytes are stolen for overhead.

garlin said:

Ethernet's normal frame size allows a payload of 1500 bytes, or the default MTU that Windows chooses. Your ISP's network equipment may use a different transmission technology that drops their frame size down, or bytes are stolen for overhead.

Click to expand...

Thanks. I believe so.

I can't remember the last time I ever had to set the MTU - with modern high speed internet connections, it is pretty irrelevant.

I take the simple view that as the default MTUs do not cause any detectable performance issues, then follow the old adage "If it ain't broke, don't fix it".

Windows tends not to have a problem because Path MTU discovery is enabled by default.

It's not the same as the Gravitational constant or the number Pi (π)


Get-NetAdapter | Where-Object {$_.Name -Match "Ethernet"} | Set-NetIPInterface -NlMtuBytes <xxxx>

@suatcini54 so with 1464 I get no packet loss but when I add 28 as you suggested and test with 1492 I get 4 packets lost, is that normal? shall I still set my router to 1492?

I have an ASUS GT-BE19000 router and my ISP uses Fibre (DHCP connection)

You should continue to increase or decrease mtu 1464 by 1 or 2 to find the best value that gives you lossless packet transmission for your internet connection, then add 28. When you add 28, the header, you do not test your system with this value. It is the header and it does not enter into the calculations.

Spartan said:

@suatcini54 so with 1464 I get no packet loss but when I add 28 as you suggested and test with 1492 I get 4 packets lost, is that normal? shall I still set my router to 1492?

I have an ASUS GT-BE19000 router and my ISP uses Fibre (DHCP connection)

Click to expand...

I get similar results. 0% loss at 1472 and 100% loss anything above that. Mine is already set to 1472 so Verizon Fios must have set it for me since I have their router. I got one at 8ms but most of them are 17ms ping replies

PMTUD (Path to MTU Discovery) should be on by default, unless it's been changed by you or a tweaking app.

Run this command, and check the PMTU value.

My result from the DF ping test is 1472 bytes, or a desired MTU of 1500 (1472 + 28).

Therefore my Windows is already doing the right thing. When PMTUD doesn't work, it's because it's been disabled or you have some weird firewall rule blocking ICMP traffic (so it can't get the proper responses). While you can hard-code the MTU value, it's best to allow Windows to figure it out.

In case your ISP switches out their router/modem, or you're roaming with a laptop on public Wi-Fi.