If you have just changed ISPs or moved house and your internet suddenly starts misbehaving the likelihood is your Maximum Transmission Unit (MTU) is set too high for your ISP. The default internet facing MTU is 1500 bytes, BUT depending on your setup, this often needs to be set much lower.
Step 1:
First check your current MTU across all your ipv4 interfaces using netsh:
netsh interface ipv4 show subinterfaces
MTU MediaSenseState Bytes In Bytes Out Interface
------ --------------- --------- --------- -------------
4294967295 1 0 0 Loopback Pseudo-Interface 1
1492 1 675 523 Local Area ConnectionAs you can see, the Local Area Connection interface is set to a 1492 bytes MTU. So how do we find out what it should be? We are going to send a fixed size Echo packet out, and tell the network not to fragment this packet. If somewhere along the line this packet is too big then this request will fail.
Next enter (if it fails then you know your MTU is too high):
ping 8.8.8.8 -f -l 1492Procedure to find optimal MTU:
For PPPoE, your Max MTU should be no more than 1492 to allow space for the 8 byte PPPoE “wrapper”. 1492 + 8 = 1500. The ping test we will be doing does not include the IP/ICMP header of 28 bytes. 1500 – 28 = 1472. Include the 8 byte PPPoE wrapper if your ISP uses PPPoE and you get 1500 – 28 – 8 = 1464.
The best value for MTU is that value just before your packets get fragmented. Add 28 to the largest packet size that does not result in fragmenting the packets (since the ping command specifies the ping packet size, not including the IP/ICMP header of 28 bytes), and this is your Max MTU setting.
The below is an automated ping sweep, that tests various packet sizes until it fails (increasing in 10 bytes per iteration):
C:\Windows\system32>for /l %i in (1360,10,1500) do @ping -n 1 -w 8.8.8.8 -l %i -f
Pinging 8.8.8.8. with 1400 bytes of data:
Reply from 8.8.8.8: bytes=1400 time=6ms TTL=64
Ping statistics for 8.8.8.8:
Packets: Sent = 1, Received = 1, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 6ms, Maximum = 6ms, Average = 6ms
Pinging 8.8.8.8 with 1401 bytes of data:
Reply from 8.8.8.8: bytes=1401 time<1ms TTL=64
Ping statistics for 8.8.8.8:
Packets: Sent = 1, Received = 1, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
Pinging 8.8.8.8 with 1402 bytes of data:
Reply from 8.8.8.8: bytes=1402 time<1ms TTL=64
Ping statistics for 8.8.8.8:
Packets: Sent = 1, Received = 1, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
Pinging 8.8.8.8 with 1403 bytes of data:
Reply from 8.8.8.8: bytes=1403 time<1ms TTL=64
Ping statistics for 8.8.8.8:
Packets: Sent = 1, Received = 1, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms Once you find the MTU, you can set it as per below:
set subinterface “Local Area Connection” mtu=1360 store=persistent