IP Fragmentation and 576 bytes !!

hi,

 

when an IP get fragmented, is the new IP MTU for the fragments get to be 576 bytes ??

 

and is this fixed number for all scenarios ?

 

unfortunately could not find a clear explanation on the blogs

Comments

  • any help guys ?

  • JoeMJoeM ✭✭✭

    Hi Oudmaster,   can you reference the blogs you are reading?

    576bytes is an RFC parameter for the "mininum MTU" that any router should be set.  RFC 791 (1981) is for Internet Protocol (IP).

     

    Below is what happens when the size of a package exceeds the IP MTU (1500+14 ethernet header in Cisco):

    I have included the actual packet capture, so that you can look at it in wireshark. Image below is not very clear here.

    Testing with ICMP using interface IP MTU of 1500:

         ping 2.2.2.2 so lo0      default = 114 = 80(icmp)+20(ip)+14(ethernet)
         ping 2.2.2.2 so lo0  size 1500
          ,,    ,,    ,,  ,,   ,,     size 2000
          ,,    ,,    ,,  ,,   ,,     size 4000
          ,,    ,,    ,,  ,,   ,,     size 6000

    image

     

     

  • JoeMJoeM ✭✭✭

    Let's breakdown the ICMP packet size 6000.  Default IP MTU is 1500.

    ping 2.2.2.2 so lo0 size 6000

    We see 1514 * 4  AND 94 * 1

              1514 - 14 (eth header) - 20 (ip header) = 1480 (actual ICMP size)

                                                                            1480 * 4 = 5920

              94 - 14(eth) - 20(IP) = 60 (last icmp packet)

     

              5920 + 60 = 5980 + 20 = size 6000

    Note that Cisco includes the original 20byte IP header in its original size.

    And if that confuses you,  try it on a Juniper device. They will calculate it a little differently.  ;-)

     

    NOW....let's look at the size 4000

    ping 2.2.2.2 so lo0 size 4000

    1514 (fragment) =  1480 + 20 + 14
    1514 (fragment) =  1480 + 20 + 14
    1054 (remaining) = 1020 + 20 + 14

    1480 + 1480 + 1020 = 3980 + 20(original IP header)= 4000

     

     

  • Pretty good but would like to add:

    The decision when to fragment is not  based on Layer 2 MTU   ,  it is based on IP MTU.  IP MTU<LAYER 2 MTU.

     EXAMPLE:

    R1#show running-config interface f0/0

    interface FastEthernet0/0

     ip address 12.12.12.1 255.255.255.0

     ip mtu 1400

     duplex full

    Layer 2 MTU is default i.e 1500




    Any packet with size>1400 will result in fragmentation i.e  it does not need to be larger than 1514 ( layer 2 MTU) , it has to be larger than the IP MTU currently set , for fragmentation to occur.
















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