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Cisco MTU loopback size

Posted on 2006-05-03
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Last Modified: 2010-05-18
Anyone knows the reason why the default
MTU size on loopback and GRE tunnel interfaces is 1514 bytes in place of the
"standard" 1500 ?
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Question by:lrinetti
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Expert Comment

by:mikebernhardt
ID: 16600157
1500 is the maximum payload size, which would include an IP packet. 1514 is the maximum frame size for an ethernet frame (not including the 4-byte FCS which makes it 1518).
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giltjr earned 750 total points
ID: 16602328
mikebernhardt is correct.  The confusing part is that some software shows you the MTU some show you the framesize minus FCS and some show you the frame size with FCS.

On loopback devices the MTU is not always limited to 1500/1514/1518, some software allows all the way up to the max of 65K.  Different software will default to different sizes.  For routers the MTU is generally set to "1500", for OS's they generally default to larger sizes.
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by:giltjr
ID: 16611635
Thanks for the points, but mikebernhardt did give the correct answer (at least in my opinion) first.  I agreed with his and expaned abit.  He should reall get the accecpted and I would be statisfied with assisted.
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