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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?
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.
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.
