for where does the mtu apply? does it apply at the layer 2 boundry or layer 3 ?

Hello,

The most common interpretation of MTU is that it is the maximum payload encapsulated inside an L2 frame. That is, if we remove the L2 header and trailer from the L2 frame, the remaining bytes is the MTU.

When you do a ping from a windows pc, you can even specify a size of zero (ping -l 0 ). This means that zero is the payload of the ICMP packet. The payload inside the L2 frame consists of the IP and the ICMP headers (with zero bytes of ICMP data).

Kind Regards,

M.

so does ethernet fragment the packet if its too big? and can layer 3 and layer 4 also fragment, where is it normally done ?

Hi

Fragmentation is done by Layer 3 device.So it is doen at L3.If the packet needs to be fragmented in between its journey to the destination it reamins fragmented till it reaches to the destination.Even though the in between links now has more MTU size it gets assemled at the destination only.

HTH,

regards,

shri :)