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Cisco MTU size

Posted on 2011-09-28
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Does the Cisco default MTU of 1500 bytes specify 1500 bytes of data payload or is it payload and headers?

Does MTU describe total size with headers or just specify the amount of data payload?
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Question by:Dragon0x40
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davorin earned 167 total points
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MTU is used for packet frame size which consists of control information (or header) and user data (or payload). It is total size.
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by:rochey2009
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Hi,

This is the payload.

For example, Ethernet can carry a payload of between 46-1500 bytes of data.
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by:davorin
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I would not agree with rochey2009. Look at this white paper http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml

Look at example 1 in the middle of the document.

MTU is the size of the datagram, which consists of header + payload
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by:rochey2009
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1500 bytes is Ethernet payload, the cisco document refers to ip header + ip payload.
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by:MAG03
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The default MTU is the total size of a packet including the header and trailer. However, if the data size passing through the interface is larger than the MTU size, the header and trailer are then removed during the fragmenting process and later readded.
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by:davorin
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And document also refers to IP MTU...
and 1500 bytes is ethernet MTU - on this I think we all agree. But we disagree if this is also the size of payload - what is the actual question...
Link to some standards document would help here..
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by:MAG03
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by:Dragon0x40
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Surfing the web for answers. Sorry it took so long!

layer 4 Transport layer  - Maximum Segment Size is 65K but since ethernet will only accept 1500 bytes payload the MSS would be most efficient for the TCP/IP stack if between 1400 and 1500 bytes.

layer 3 Network - Maximum packet size is 1500 bytes for ethernet and that includes the payload and IP header

layer 2 Data link - Maximum frame size is determined by manufacturer but wireshark normally shows 1514 bytes. 1500 for payload + 6 for source mac + 6 for destination mac + 2 for type/length

Wireshark does not show preamble 8 bytes and FCS 4 bytes so my guess is that on the physical wire you could see frames up to 1526 bytes or on a dot1q trunk 1530 with the 4 byte dot1q tag.

I think that is what everyone was trying to say in different ways.

What do you think?

http://stackoverflow.com/questions/2613734/maximum-packet-size-for-a-tcp-connection
http://wiki.wireshark.org/Ethernet
http://wiki.wireshark.org/MTU
http://en.wikipedia.org/wiki/Ethernet_frame

my windows computer:

C:\Users\xxxx>ping 10.1.1.1 -f -l 1472

Pinging 10.1.1.1 with 1472 bytes of data:
Reply from 10.1.1.1: bytes=1472 time=3ms TTL=254
Reply from 10.1.1.1: bytes=1472 time=3ms TTL=254
Reply from 10.1.1.1: bytes=1472 time=3ms TTL=254
Reply from 10.1.1.1: bytes=1472 time=3ms TTL=254

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