MTU

"...the MTU is the size of the largest Layer 3 packet that can sit inside a frame's data field. For data-link protocols, Ethernet included, the MTU is 1500 bytes."

- Odom, CCNA ICND1 Official Exam Certification Guide, PG #144

"It is important to remember that this Layer 2 MTU affects all protocols that use the interface and not just IP. This is the size of the largest Layer 2 packet that the router can send through the interface."

- Cisco IOS Cookbook, 2nd Ed

"In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet or frame that a given layer of a communications protocol can pass onwards."

http://en.wikipedia.org/wiki/Maximum_transmission_unit

Ok, who's lying? Is the MTU the size of the payload within a layer 2 frame or the size of the entire frame (payload + header) ?

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Comments

tiersten

Wikipedia = Unreliable. Don't trust it.

GT-Rob

Its the size of the frame. MTU is referring to Layer 2

I think :P

wat08

Yeah, I've always thought it was the size of the frame. Here's another source:

"The maximum amount of data that can be grabbed and placed in a segment is limited by the Maximum Segment Size (MSS). The MSS is typically set by first determining the length of the largest link-layer frame that can be sent by the local sending host (the so-called maximum transmission unit, MTU)."

A Top-Down Approach to Computer Networking, Kurose & Ross, PG # 242-243

So I've pretty much had it with Odom's inaccuracies and wishy-washy explanations...

dtlokee

MSS is often used to describe the largest unit of data that is handed down from the transport layer to the Network layer. I have seen MTU used in reference to both L2 and L3. ultimately the largest frame that can be transmitted is dependent on the media.

RFC791 wrote:

The maximum sized datagram that can be transmitted through the next network is called the maximum transmission unit (MTU)

MTU is used in terms of IP therefore L3. I don't think you can assign it to a singular layer. It's kinda like calling it a "packet" when "datagram" is correct.