12-26-2011 08:01 PM - edited 03-07-2019 04:03 AM
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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12-27-2011 10:36 AM
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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.)
12-26-2011 08:54 PM
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
HTH
12-27-2011 10:36 AM
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.)
01-10-2012 04:20 AM
thanks.
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