Confused about the MTU?

nixdaemon000

Guys,
I am really confused about the MTU thing. Is it the maximum data that can be sent through a link including Ethernet frame header + data payload (including IP headers) or MTU is just a maximum data size excluding Ethernet frame header (only the data payload in the frame without the ethernet header)?

Slowhand

The MTU includes the header and payload. Since both take up bandwidth, they're both included as a whole part of any datagram. The header and payload aren't considered seperate entities until a router or switch begins disassembling a packet or frame to read it, and presumably forward it.

ULWiz

I personally would not be too concerned about this for the Network+ exam its not going to touch this at all.

shednik

Slowhand wrote:

The MTU includes the header and payload. Since both take up bandwidth, they're both included as a whole part of any datagram. The header and payload aren't considered seperate entities until a router or switch begins disassembling a packet or frame to read it, and presumably forward it.

To extend on what slowhand said here's a question i had from my grad school course on it that may explain it well

Question wrote:

Consider sending a 3000-byte diagram into a link that has an MTU of 500 bytes. Suppose the original datagram is stamped with the identification number 422. How many fragments are generated? What are their characteristics?

The answer I had was:

Since the MTU is set at 500 bytes and the datagram is 3000 bytes we will need to break it down into 480 bytes for each fragment leaving 20 bytes for the IP header. So there will end up being 7 fragments where all but the last will be 500 bytes in size. They will all have an id number of 422 and the first six will be set with the flag equal to 1 and the last equal to 0.

hope this helps...but like it was said I don't remember being tested much on MTU on N+

Slowhand

I'm going to agree, and say that you won't have to worry about things like the breakdown of TCP/IP headers or the MTU on Network+, that's more of a CCNA (and beyond) topic. However, I think it's great that you're asking these types of questions. Learning things beyond the scope of the exam is never going to hurt you, (unless you trade that for studying the exam topics,) and I think that you'll be a better network admin for it. Don't be afraid to read up on things that your study guides just touch on, it's all good stuff.