In following topology, How does the my computer calculates the MTU of (lan cable1) to fragment the packets? Does switches have any MTU limit? If yes then, According which thing (lan cable2 or SWITCH3) ROUTER1 fragments the packet?

On what basis MTU is calculated?

Me <--(lan cable1)--> ROUTER1 <----(lan cable2)----> SWITCH3 <----(lan cable3)----> ROUTER4 <---(lan cable4)---> SERVER(1 GB file)

Thanks in advanced.

  • 1
    The answer depends on a lot of things, e.g. the transport protocol which the application uses. If it is UDP the application needs to do more work than if it is UDP. Your question is really too broad to answer. You should edit it to narrow the focus. We can't give you a whole semester of network theory.
    – Ron Maupin
    Commented May 21, 2016 at 17:54
  • 2
    You first need to learn how TCP works. Once you have a basic understanding of that, you can probably answer your own question. There are many good sources to read or watch on you tube.
    – Ron Trunk
    Commented May 21, 2016 at 21:40
  • 1
    Path MTU Discovery
    – Ricky
    Commented May 23, 2016 at 19:24

1 Answer 1


MTU (Maximum transmission unit ) defines the largest size of packets (layer 3) that an interface can transmit without the need to fragment. The MTU of an Ethernet connection by default = 1500 bytes. If a system sends packets over an Ethernet network that are larger than 1500 bytes, the data will be fragmented into smaller packets.

To calculate: MTU = MSS + 40 where:

MSS (Maximum segment size) is a parameter of the options field of the TCP header that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment and 40 = IP ( 20 bytes) + TCP ( 20 bytes). If you set the MTU of the interface to 1492, you should see the MSS to 1452.

The TCP/IP doesn't limit the size of the data of the segment; Ethernet defines the maximum size of the dates before the encapsulation; IP defines the maximum size of the dates after the encapsulation; At layer 2 OSI the minimum size of a frame (Ethernet) could be 64 bytes and the maximum 1518 bytes.

So I think you should analyse the problem in terms of protocols, not devices.

  • Actually, 20 bytes is the minimum IPv4 header size, and depending on IPv4 options, it could be as large as 60 bytes. IPv6 has a fixed header size of 40 bytes, but it could have extension headers. The maximum total length of an IPv4 packet is 65,536 bytes, and the maximum IPv6 packet payload is 65,536 bytes. This will limit the size of a TCP segment.
    – Ron Maupin
    Commented May 31, 2016 at 14:03

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