MTU - Interface MTU vs IP MTU

When discussing the Maximum Transmission Unit or MTU, it can often be confusing when addressing the MTU at various layers of the OSI Model.

For example, the interface MTU differs from the IP MTU. Similarly, this differs from the MSS configuration for TCP.

Take a look at this diagram showing the various default MTU sizes:

mtu-header-sizes.png

If the interface MTU is the same as the IP MTU, why are there two such size designations?

We must keep in mind that the definition of each corresponds to the layer in the OSI model in which that entity exists.

The interface MTU, sometimes referred to as the Layer 2 MTU, doesn’t include the Ethernet header. The MTU that is configured on an L2 interface of a switch, refers only to the payload of the Ethernet frame.

At Layer 3, the IP MTU includes the IP header and the TCP header as well, and all the contents within the IP payload.

So it makes sense that, as defined above, the L2 MTU is equal to the L3 MTU. If we imagine the Ethernet frame, the IP packet, and the TCP segment as boxes that fit into one another, we can think about MTU measurements as follows:

  • The L2 MTU is measured as the capacity of the box that will encapsulate the upper layer packet. It’s a measurement of the INSIDE of the box, in other words, the size of the payload it can carry, without measuring the actual dimensions of the box itself. So we leave out the box itself which is the additional 14 bytes of the Ethernet header.
  • The L3 MTU is measured as the size of the OUTSIDE of the box at the IP layer, that is, the IP payload AND the header. It includes the dimensions of the box, so it has a value of 1500 which comes from the 1480 for the payload and 20 for the IP header.

https://networklessons.com/ip-routing/pppoe-mtu-troubleshooting-cisco-ios

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