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New Member

what are differences between MTU and maximum frame size

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.

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Super Bronze

what are differences between MTU and maximum frame size

Disclaimer

The     Author of this posting offers the information contained within this     posting without consideration and with the reader's understanding  that    there's no implied or expressed suitability or fitness for any   purpose.   Information provided is for informational purposes only and   should not   be construed as rendering professional advice of any kind.   Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In     no event shall Author be liable for any damages whatsoever   (including,   without limitation, damages for loss of use, data or   profit) arising  out  of the use or inability to use the posting's   information even if  Author  has been advised of the possibility of  such  damage.

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.)

3 REPLIES
VIP Super Bronze

what are differences between MTU and maximum frame size

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

Super Bronze

what are differences between MTU and maximum frame size

Disclaimer

The     Author of this posting offers the information contained within this     posting without consideration and with the reader's understanding  that    there's no implied or expressed suitability or fitness for any   purpose.   Information provided is for informational purposes only and   should not   be construed as rendering professional advice of any kind.   Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In     no event shall Author be liable for any damages whatsoever   (including,   without limitation, damages for loss of use, data or   profit) arising  out  of the use or inability to use the posting's   information even if  Author  has been advised of the possibility of  such  damage.

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.)

New Member

what are differences between MTU and maximum frame size

thanks.

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