![]() Going back to our original two routers, we can see that as long as our IP packets doesn’t exceed 1500 then there should be no fragmentation. In total, this comes to a frame size of 1522 bytes. Finally, there is the Frame Check Sequence/Cyclic Redundancy Check (4 bytes). Next is the Ethertype/Length field (2 bytes), followed by the payload (upto 1500 bytes). Then follows an optional 4 bytes reserved for VLAN trunking. We can see the Ethernet frame comprises of a MAC destination address and MAC source address (6 bytes each). If the IP MTU is 1500 what is the total size of the Ethernet frame? To understand this we need to look at the Ethernet Frame structure. R2#show ip interface gigabitEthernet 0/0 | i MTUīy default the IP MTU is set to the maximum possible value, but can be lowered if required: R1#conf t We can see the current value of the IP MTU using the show ip interface command R1#show ip interface gigabitEthernet 0/0 | i MTU Note this is not the Ethernet frame size. This shows the maximum possible IP MTU the interface will allow. R2#show interfaces gigabitEthernet 0/0 | i MTU R1#show interfaces gigabitEthernet 0/0 | i MTU If we look at the MTU (Maximum Transmission Unit) set on the interfaces we see the following. The configuration of the interfaces is as follows: R1#show run int gigabitEthernet 0/0 R1 and R2 are connected on an Ethernet segment. To understand how it works consider the following back-to-back network. IPv4 Fragmentation and Reassembly is an important feature of today’s networks.
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