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Diagnose and Fix MTU Size Mismatch Problems

Incorrect Maximum Transmission Unit (MTU) sizes cause packet fragmentation, resulting in degraded speeds, dropped connections, and inaccessible websites.

mtupacket-lossvpnping

Confirmed working on: Windows 10 · Windows 11 · Routers · VPNs

Before you begin

Match the environmentUse this guide only for Windows 10, Windows 11, Routers, VPNs or a clearly compatible setup.
Protect recoveryBack up important data and save encryption or account recovery keys before changing system state.
Know when to stopThis guide includes advanced changes. If the device contains irreplaceable data or belongs to an employer, stop and use a qualified technician.

Problem overview

Your internet connection is unstable, particularly when using a VPN or connecting to specific servers. Packets are being fragmented or dropped because the configured MTU size on your device exceeds the capacity of the network path.

Common causes

  • Using a VPN tunnel without accounting for encryption overhead
  • Your ISP uses PPPoE, which requires an MTU of 1492 instead of 1500
  • Misconfigured router WAN settings
  • Path MTU Discovery (PMTUD) failing due to blocked ICMP packets

Solution 1: Find the optimal MTU size

  1. Open Command Prompt as administrator.
  2. Run a ping command with the don’t fragment (-f) flag and a payload size (-l) of 1472 (1500 minus 28 bytes of IP/ICMP overhead): 
    ping google.com -f -l 1472
  3. If you get a reply, your network supports an MTU of 1500.
  4. If you get “Packet needs to be fragmented but DF set,” lower the payload size by 10 (e.g., 1462) and try again.
  5. Keep lowering the number until the ping succeeds.
  6. Once it succeeds, add 28 to that number to find your optimal MTU. (e.g., if 1440 succeeds, 1440 + 28 = 1468).

Solution 2: Set the MTU on Windows

  1. Open Command Prompt as admin.
  2. Find the exact name of your network adapter by running: 
    netsh interface ipv4 show subinterfaces
  3. Change the MTU size using the optimal number you calculated: 
    netsh interface ipv4 set subinterface "Wi-Fi" mtu=1468 store=persistent
  4. Replace "Wi-Fi" with your adapter’s actual name. Restart your PC.
Expert Tip

If you are fixing a VPN connection, make sure you apply the MTU change to the virtual VPN adapter (e.g., TAP-Windows Adapter), not just your physical Wi-Fi or Ethernet adapter.

Solution 3: Change the MTU on your router

  1. Log into your router’s admin panel.
  2. Navigate to the WAN, Internet, or Network settings.
  3. Look for the MTU Size field.
  4. If your ISP uses PPPoE (common with DSL or fiber), set it to 1492.
  5. If you calculated a different optimal MTU, enter it here.
  6. Save the settings and reboot the router.

Solution 4: Enable Path MTU Discovery (PMTUD)

  1. PMTUD automatically negotiates the correct MTU size, but it requires ICMP packets to work.
  2. Ensure you have not aggressively blocked all ICMP traffic in your router’s firewall.
  3. Check your Windows Registry to ensure PMTUD is enabled (it is by default). Press Windows Key + R, type regedit, and navigate to: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters
  4. If EnablePMTUDiscovery exists, ensure its value is set to 1.
Critical

Do not set your MTU arbitrarily low “just to be safe.” A very low MTU drastically increases packet overhead and will severely reduce your overall network throughput.

Confirm the result

  1. Repeat the exact action that originally triggered the problem.
  2. Confirm the original error or symptom is gone and no new warning has appeared.
  3. If the guide changed a driver, service, package, or system setting, restart once and test again.
  4. If the result is worse or unexpected, stop. Reverse only the last change using its documented restore option; if none is documented, use your backup or qualified support.

Questions about this fix

What is the standard MTU size?

The standard MTU for most Ethernet and Wi-Fi networks is 1500 bytes. However, overhead from PPPoE or VPNs requires smaller MTU sizes to prevent fragmentation.

How do I know if my MTU is wrong?

If large downloads fail, VPN connections drop, or specific websites refuse to load while others work fine, MTU fragmentation is a likely culprit.

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