MTU of a loopback is 1514.......why?


Rack1R2# sh int lo 0

Loopback0 is up, line protocol is up

  Hardware is Loopback

  Internet address is 150.1.2.2/24

  MTU 1514 bytes, BW 8000000 Kbit, DLY 5000 usec,

     reliability 255/255, txload 1/255, rxload 1/255



why is a loopbacks mtu 1514 ?

Comments

  • I think for the loopback interfaces IOS considers the L2 header size in the MTU but for physical ethernet interface it excludes it so 1500 for physical and 1514 for Loopback. Just my thought..:)

     

    Regards,

    Arun

  • Yes the loopback has the encapsulation LOOPBACK. and this encapsulation oviously is no real encapsulation like ethernet which lets ARUNs message make sense.

    Interesting for me would be the Bandwidth...does anyone know why its 8Gbps?

    Regards!

  • IOS considers the L2 header size in the MTU but for physical ethernet interface it excludes it so 1500 for physical and 1514 for Loopback

    Thats fine but the question was asked Why MTU 1514 for loopback?

  • IOS considers the L2 header size in the MTU but for physical ethernet interface it excludes it so 1500 for physical and 1514 for Loopback

    Thats fine but the question was asked Why MTU 1514 for loopback?

     

    He just answered that. 1500+L2 headers that aren't counted on a regular interface

  • sorry, why would we need to care about loopback interface's MTU?

  • He just answered that. 1500+L2 headers that aren't counted on a regular interface

    How loopback MTU 1514 ----- 1500+L2

    But why is loopback 1514 is still unanswered. Loopback never sends out packets from its interface to anyone it ultimately sends out packets from physical interface which always count L2=14byes and CRC=4byes.It might be used for testing when packets are looped back from the local interfaces for the purpose of checking the NIC connectivity status locally but i am not sure exactly how?

    So my question remain same why loopback has MTU 1514?

  • It might be used for testing when packets are looped back from the local interfaces for the purpose of checking the NIC connectivity status locally but i am not sure exactly how?


    The loopback interface discussed here cannot be used for loopback test of physical interface. Check out this link for one example of interface loopback test: http://www.cisco.com/en/US/tech/tk713/tk628/technologies_tech_note09186a00800a754b.shtml.

  • is this MTU Really make any difference ... Out there in Exam or real world 

     

     

  • You might be concerned about the MTU if you are using a loopback interface as part of a hairpin NAT process. (This would be more likely in a PIX or ASA, though.) I don't see a way to change the loopback mtu on the 3640 I have handy - can change mtu on the serial and fast ethernet interfaces.

    This could apply to policy routing through a loopback interface, though - if you are routing a tunnel via the loopback, you would need to know that the size the of packets don't exceed the MTU.

    He just answered that. 1500+L2 headers that aren't counted on a regular interface

    How loopback MTU 1514 ----- 1500+L2

    But why is loopback 1514 is still unanswered. Loopback never sends out packets from its interface to anyone it ultimately sends out packets from physical interface which always count L2=14byes and CRC=4byes.It might be used for testing when packets are looped back from the local interfaces for the purpose of checking the NIC connectivity status locally but i am not sure exactly how?

    So my question remain same why loopback has MTU 1514?

     

  • Don't think we can change the MTU value on the Loopback interface, it's fixed 1514, as to why 1514 for loopbacks I do not no, may be the architect who designed IOS might know.

    Regards,

    Arun

  • Don't think we can change the MTU value on the Loopback interface, it's fixed 1514, as to why 1514 for loopbacks I do not no, may be the architect who designed IOS might know.

    Regards,

    Arun


     

    Rack1R1(config)#int loopback 0

    Rack1R1(config-if)#ip mtu

    Rack1R1(config-if)#ip mtu ?

      <68-1514>  MTU (bytes)




  • Oh wow, thank you ToxicAvenginator, I did not try that :D

    Regards,

    Arun

  • Hi all,

       As many other cases, i find this to be just cosmetic, but still with some reasons behind it. You also get the same default MTU of 1514 for tunnel interfaces, i think regardless of the encapsulation type. Now the difference between default Ethenet MTU of 1500 and default loopback MTU of 1514, it is 14, which is the size of Ethernet header (trailer is separate). Now as from a loopback/tunnel interface there is no ethernet header when the frame exists the interface, it makes sense from an architectural pespective that MTU is 1514.

    Good luck with your studies!

  • Oh wow, thank you ToxicAvenginator, I did not try that :D

    Regards,

    Arun

    you r quite welcome mon ami but i really did not do anything that you would not have eventually done!

     

     

  • Hi all,

       As many other cases, i find this to be just cosmetic, but still with some reasons behind it. You also get the same default MTU of 1514 for tunnel interfaces, i think regardless of the encapsulation type. Now the difference between default Ethenet MTU of 1500 and default loopback MTU of 1514, it is 14, which is the size of Ethernet header (trailer is separate). Now as from a loopback/tunnel interface there is no ethernet header when the frame exists the interface, it makes sense from an architectural pespective that MTU is 1514.

    Good luck with your studies!

    Thankyou!

     

    the point i asked this question ws..is there ANY reason i need to remember this for my CCIE?

    if not i'm not going to bother remembering...

    such trivia also fun and interesting..i can indulge to my hearts content AFTER i have a #

  • Hi Toxic,

      Don't think everything in #/numbers, just focuse on learning as much as you can, even stuff that on first look has no scope or meaning, it may further help you!

    Good luck with your studies!

  • Hi Toxic,

      Don't think everything in #/numbers, just focuse on learning as much as you can, even stuff that on first look has no scope or meaning, it may further help you!

    Good luck with your studies!

    I'm trying...

     

    but...there is only limited space/quanta of time upstairs [:S]

     

    so i prioritize

     

  • but...there is only limited space/quanta of time upstairs Tongue Tied

     

     


    Hahaha...I totally agree with you man. I don't think we really need to care about loopback interfaces' MTU for the exam. This is only based on my limited exp.

  • but...there is only limited space/quanta of time upstairs Tongue Tied

     

     


    Hahaha...I totally agree with you man. I don't think we really need to care about loopback interfaces' MTU for the exam. This is only based on my limited exp.

    I'm VERY glad that christian replied in this thread...if only to make me realize that i dont need to worry about its MTU.

    if there ws something to remember..as it might have related to the exam i'm sure he would have pointed us in the right direction

     

     

    having said that..i dont have a good memory...at least compared to my friends....so the less i have to remember..the better.

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