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    Fibre mtu size

    This is a discussion on Fibre mtu size within the Sky Broadband (Fibre) Help forums, part of the Sky Broadband help and support category; Hi guys. What's the best mtu size for sky fibre? 1492 or 1500? Thanks Keith...

    1. #1
      kman28's Avatar
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      Fibre mtu size

      Hi guys.

      What's the best mtu size for sky fibre? 1492 or 1500?

      Thanks Keith
      Sky Fiber 40/10 Connection - DL 39998 - UL 10000


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    3. #2
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      Re: Fibre mtu size

      Quote Originally Posted by kman28 View Post
      Hi guys.

      What's the best mtu size for sky fibre? 1492 or 1500?

      Thanks Keith
      Good question! The default setting on my Sky Hub is 1500 and I haven't changed it and my connection is "great". But the help box on the relevant page in the Sky Hub's UI states:

      "The normal MTU (Maximum Transmit Unit) value for most Ethernet networks is 1500 Bytes, or 1492 Bytes for PPPoE connections. For some ISPs you may need to reduce the MTU ,but this is rarely required, and should not be done unless you are sure it is necessary for your ISP connection."
      ++ speedyrite ... powered by NOW Broadband from June 2018 ++
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    4. #3
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      Re: Fibre mtu size

      1500 byte packets are supported, I've not tried setting the MTU higher on the interface.

      Code:
      admin@MikroTik] > ping google.com size=1500 do-not-fragment  
      HOST                                     SIZE TTL TIME  STATUS                   
      173.194.34.160                           1500  59 8ms  
      173.194.34.160                           1500  59 8ms  
      173.194.34.160                           1500  59 8ms  
      173.194.34.160                           1500  59 8ms  
      173.194.34.160                           1500  59 8ms  
      173.194.34.160                           1500  59 8ms  
      173.194.34.160                           1500  59 8ms  
      sent=7 received=7 packet-loss=0% min-rtt=8ms avg-rtt=8ms max-rtt=8ms


      Code:
      [admin@MikroTik] > ping google.com size=1501 do-not-fragment  
      HOST                                     SIZE TTL TIME  STATUS                   
                                                              packet too large and c...
      94.6.0.54                                 576  64 4ms   fragmentation needed a...
                                                              packet too large and c...
      94.6.0.54                                 576  64 3ms   fragmentation needed a...
                                                              packet too large and c...
      94.6.0.54                                 576  64 1ms   fragmentation needed a...
                                                              packet too large and c...
      94.6.0.54                                 576  64 1ms   fragmentation needed a...
          sent=4 received=0 packet-loss=100%
      Edit:

      Nope it's definitely 1500 setting a higher MTU on your interface won't achieve anything (Well other than perhaps breaking things)
      speedyrite and Angel_Rex like this.

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      Re: Fibre mtu size

      While someone has brought up a question regarding MTU I will jump in and ask.

      I want to be able to change my MTU size but which Router mode setting do I use as currently it is set to Auto so the MTU settings are hidden.

      I know I need to switch it from Auto to either ADSL or WANoE to be able to access it but Im not too sure which the Auto uses for FTTC?

      Hope you can understand. Thanks.

    6. #5
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      Re: Fibre mtu size

      Quote Originally Posted by Kumper View Post
      While someone has brought up a question regarding MTU I will jump in and ask.

      I want to be able to change my MTU size but which Router mode setting do I use as currently it is set to Auto so the MTU settings are hidden.

      I know I need to switch it from Auto to either ADSL or WANoE to be able to access it but Im not too sure which the Auto uses for FTTC?

      Hope you can understand. Thanks.
      If you set it to WANoE the hidden MTU settings are revealed. This is also the best setting for sky fibre because if you have to do a reboot, it will reconnect quicker than AUTO.

      Sent from my ST18i
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      Re: Fibre mtu size

      Quote Originally Posted by Fryingpan77 View Post
      If you set it to WANoE the hidden MTU settings are revealed. This is also the best setting for sky fibre because if you have to do a reboot, it will reconnect quicker than AUTO.

      Sent from my ST18i
      Thanks .

    8. #7
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      Re: Fibre mtu size

      Although changing the setting may reveal the option to Change the MTU the default of 1500 seems fine, Even if you set the MTU higher you wont be able to send packets larger than 1500bytes anyway.

      Not sure if the Openreach Modem or the Network is the limitation there but either way...

    9. #8
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      Re: Fibre mtu size

      Quote Originally Posted by dragon2611 View Post
      Although changing the setting may reveal the option to Change the MTU the default of 1500 seems fine, Even if you set the MTU higher you wont be able to send packets larger than 1500bytes anyway.

      Not sure if the Openreach Modem or the Network is the limitation there but either way...
      Im not doing it to be able to change the MTU figure higher, only messing around with changing abit lower thats all.

    10. #9
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      Re: Fibre mtu size

      Isn't 1500 bytes the maximum allowable packet size? The MTU can be lower, some use as low a 1400, TalkTalk recommend 1432. Sky use 1472+28 for overheads = 1500. Anything higher than this will fail and anything much lower will require extra packets to be sent, thus slowing down the connection.

      !500 is fine for ADSL but Fibre may be different. If you want to check open a Command Prompt and type ping -f -l [mtu number] wwwdotskydotcom You want the highest figure which will does not return a packet loss +28 for overheads. Miss the brackets and replace dot with ..

      I picked this up from The Scream some time ago, but can't find the original page to reference.

      I've found the original page, there is other info there too which you may find handy. Here it is for those who are interested.-
      www.the-scream.co.uk/html/resources/adsltips.html
      Last edited by Isitme; 08-10-12 at 09:22 PM. Reason: added url
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    12. #10
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      Re: Fibre mtu size

      Quote Originally Posted by Kumper View Post
      Im not doing it to be able to change the MTU figure higher, only messing around with changing abit lower thats all.
      Whilst you can do that it will just cause unnecessary packet fragmentation requiring a higher number of packets to be sent to transfer the same amount of information.

      If the network can handle 1500 Byte packets then I don't see any reason to set the MTU any lower unless you have a specific problem you are trying to overcome?

     

     
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