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piplowe
Contributor
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Message 1 of 13

MTU Low - 1272 works everything else above that fragments

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Hi

 

Is my MTU setting to low ? See results below

 

fast path enabled

 

I've tried my HH3 and a draytek 2820n router to test.

 

HH3 - MTU set to  factory default

Draytek - MTU set to 1492 on the draytek

 

 

MTU Size     Status
1300:      Fragmented!
100:      ok
200:      ok
300:      ok
400:      ok
500:      ok
600:      ok
700:      ok
800:      ok
900:      ok
1000:      ok
1100:      ok
1200:      ok
1300:      Fragmented!
1200:      ok
1250:      ok
1300:      Fragmented!
1250:      ok
1260:      ok
1270:      ok
1280:      Fragmented!
1271:      ok
1272:      ok
1273:      Fragmented!
=====================================================
Max MTU size that doesn't fragment: 1272 + 28 bytes = 1300

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Reed_Richards
Aspiring Expert
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Message 2 of 13

Re: MTU Low - 1272 works everything else above that fragments

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If 1492 does not work then you are in unusual sitiation.  I'm not sure I beleive your test results; what happens if you try 1492?  Does your internet connection stop working or slow to a crawl?

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Distinguished Sage
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Message 3 of 13

Re: MTU Low - 1272 works everything else above that fragments

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BT use an MTU OF 1500on ADSL and 1492 on fibre
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piplowe
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Message 4 of 13

Re: MTU Low - 1272 works everything else above that fragments

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Thanks for the response.

 

I found an old HH2 and tested that also and I got th same result.  As far as I know the MTU cannot be set on the HH2 and HH3

 

I was using a MTU Test Software to test so for piece of mind I went down the comand prompt route and saw the same thing.

 

C:\Users\Philip>ping www.google.com -f -l 1272

Pinging www.google.com [173.194.34.147] with 1272 bytes of data:
Reply from 173.194.34.147: bytes=1272 time=233ms TTL=52
Reply from 173.194.34.147: bytes=1272 time=37ms TTL=52
Reply from 173.194.34.147: bytes=1272 time=37ms TTL=52
Reply from 173.194.34.147: bytes=1272 time=36ms TTL=52

 

Any ideas


Here is the whole test


C:\Users\Philip>ping www.google.com -f -l 1492

Pinging www.google.com [173.194.34.147] with 1492 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1482

Pinging www.google.com [173.194.34.147] with 1482 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1472

Pinging www.google.com [173.194.34.147] with 1472 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1462

Pinging www.google.com [173.194.34.147] with 1462 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1442

Pinging www.google.com [173.194.34.147] with 1442 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1432

Pinging www.google.com [173.194.34.147] with 1432 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1402

Pinging www.google.com [173.194.34.147] with 1402 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1352

Pinging www.google.com [173.194.34.147] with 1352 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1302

Pinging www.google.com [173.194.34.147] with 1302 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1292

Pinging www.google.com [173.194.34.147] with 1292 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1282

Pinging www.google.com [173.194.34.147] with 1282 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Philip>ping www.google.com -f -l 1272

Pinging www.google.com [173.194.34.147] with 1272 bytes of data:
Reply from 173.194.34.147: bytes=1272 time=233ms TTL=52
Reply from 173.194.34.147: bytes=1272 time=37ms TTL=52
Reply from 173.194.34.147: bytes=1272 time=37ms TTL=52
Reply from 173.194.34.147: bytes=1272 time=36ms TTL=52

Ping statistics for 173.194.34.147:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 36ms, Maximum = 233ms, Average = 85ms

 

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Reed_Richards
Aspiring Expert
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Message 5 of 13

Re: MTU Low - 1272 works everything else above that fragments

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FYI, I tried the same test on my own computer (I'm not with BT for my broadband and I use a Netgear router).

 

The largest packet size I could use was 1472.  Above that I got the "Packet needs to be fragmented ..." message.

 

Then I used:

netsh interface ipv4 show subinterfaces

 

which told me the MTU on my LAN is 1500.  It is also set to 1500 for the WAN on my router.    

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piplowe
Contributor
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Message 6 of 13

Re: MTU Low - 1272 works everything else above that fragments

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I still get 1300 ? 

 

MTU  MediaSenseState   Bytes In  Bytes Out  Interface
------  ---------------  ---------  ---------  -------------
  1300                1    6789021    1943397  WiFi
  1300                5          0      14760  Local Area Connection* 12
  1300                5          0          0  Ethernet

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piplowe
Contributor
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Message 7 of 13

Re: MTU Low - 1272 works everything else above that fragments

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I also get this on speedguide.net

 

TCP options string = 020404ec0103030201010402
MTU = 1300
MTU is not fully optimized for broadband. Consider increasing your MTU to 1500 for better throughput. If you are using a router, it could be limiting your MTU regardless of Registry settings.
MSS = 1260
MSS is not optimized for broadband. Consider increasing your MTU value.
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Distinguished Sage
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Message 8 of 13

Re: MTU Low - 1272 works everything else above that fragments

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can you web browse and use the internet for all other functions when using the home hub
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Reed_Richards
Aspiring Expert
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Message 9 of 13

Re: MTU Low - 1272 works everything else above that fragments

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I think that the fact that the MTU on your computer is set to 1300 explains why only up to 1272 works without packet fragmentation.

 

Use:

netsh interface ipv4 set subinterface "Local Area Connection" mtu=1500 store=persistent

 

to set it to 1500 (which is the correct value for Ethernet).

 

piplowe
Contributor
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Message 10 of 13

Re: MTU Low - 1272 works everything else above that fragments

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Thank you

 

That's exactly what it was, I checked the same on another computer.

 

Not even sure how it got lowered in the first place

It's a windows 8 pc and I've had it from new.

 

I've got 2 VMware Lan adapters on there and they are set to 1500

 

Very odd.

 

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