what are differences between MTU and maximum frame size

[dannan lin]

hi:

i am confused between these two concepts.

CISCO said "The maximum frame size is also known as the maximum transmission unit, or MTU. When a frame is larger than the MTU, it is broken down, or fragmented, into smaller pieces by the Layer 3 protocol to accommodate the MTU of the network."

"http://www.ciscopress.com/articles/article.asp?p=357103&seqNum=3"

but from what i know MTU is somwthing to do with the IP packets. it indicates the maximum packet size that can pass the port at one time. 

frame must add other information such as eathernet heade and vlan .

frame is for layer2 and MTU is the concept for layer 3 aren't they?  how can these two be the same?

I also want to know how to check maximum frame size that is allowed on a switch ?

thanks in advance.

[Reza Sharifi]

Hi,

MTU is layer-1 and not then 2 or 3.

MTU is the maximum size that the physical interface can transmit

The supported frame size varies by line card:

Have a look at this doc. It goes over the frame size based on the chassis and line card

http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml#topic

HTH

[Joseph W. Doherty · Accepted solution]

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Posting

CISCO said "The maximum frame size is also known as the maximum transmission unit, or MTU. When a frame is larger than the MTU, it is broken down, or fragmented, into smaller pieces by the Layer 3 protocol to accommodate the MTU of the network."

Yes, that is confusing.  It might be better if worded "The maximum frame size is also known as the maximum transmission  unit, or MTU. When a packet is larger than the MTU can contain, it (the packet) is broken down, or  fragmented, into smaller pieces by the Layer 3 protocol to accommodate  the MTU of the network."

Realize frames are destroyed and newly build at every L3 hop.  (NB: L2 devices should all have the same MTU for a common shared logical segment.  It's possible they don't, for example a translation bridge between Token Ring and Ethernet, but then MTU can be an issue as pure L2 doesn't fragment too large frames but discards them.)

[dannan lin]

thanks.