Let's say that we have two PC's and one router between them:

PC1 ----------- Router ---------- PC2

PC 1 MTU is 400 bytes PC 2 MTU is 1500 bytes

We send a packet from PC1(400) to PC2(1500)

The question is: Will there be any packet fragmentation/byte excess - how the router will act in this situation?

In my opinion - no, but shouldn't it fill the data with zeros, or something?

Also, how the fragmentation works, if we switch the MTU's between two of them?

  • 3
    MTU is the maximum size of a packet, not "all packets are this size". (ethernet does, however, have a minimum frame length of 64bytes. A frame smaller than that will be padded with nulls)
    – Ricky
    Feb 2, 2016 at 21:19

2 Answers 2


PC1 can send 400 byte packets to PC2. No fragmentation.

When PC2 replies, one of two things can happen:

If the router interface to PC1 has an MTU of 400, then the router will fragment the packets.

If the router interface is set to 1500, it will not fragment, and PC1 will drop the packets.

  • If the router interface is set to 1500, it will not fragment, and PC1 will drop the packets. << What do you mean by dropping packets when the MTU is 1500 on the first PC? Why would it drop them? MTU would be equal. Feb 2, 2016 at 17:11
  • If the router's MTU is 1500, but PC1 is 400, then when PC2 sends a (large) packet to PC1, PC1 will drop it.
    – Ron Trunk
    Feb 2, 2016 at 17:53
  • Any packet larger than the MTU is an error ("oversize frame"), and will be dropped.
    – Ricky
    Feb 2, 2016 at 21:21

Going from a small MTU to a larger MTU will not fragment anything, nor will it cause any padding unless the layer-2 protocol has a minimum frame or payload size which is larger than the MTU. Remember that the "M" in MTU is for "Maximum," not "Minimum." Ethernet has a minimum frame size of 64 bytes and a minimum payload size of 46 bytes, so using ethernet will not cause any padding in the ethernet payload, and IP doesn't care so the packets will not be modified.

  • Why do we always answer at the same time? ;-p
    – Ron Trunk
    Feb 2, 2016 at 16:49
  • I don't know. I guess it's a Ron thing... :)
    – Ron Maupin
    Feb 2, 2016 at 16:50

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