MTU newb questions

[pdub206]

Hi all,

I've begun studying the TCP/IP protocol more in depth recently and found that I'm still lacking a bit in understanding of some aspects of MTU.

For example, if I have a Jumbo MTU set to 9000, will it accept all sizes below 9000 without fragmentation? 9000 is simply the point where it says "I now must fragment"?

Another question related to this -- fragmentation increases CPU usage normally, so does increasing the MTU size to 9000 reduce CPU load in environments where larger than normal MTU is used?

I've also read that in some cases we would want to -reduce- the MTU size to accomodate for some protocols, ie. GRE/IPSec tunneling.  Why is that necessary?

[Venkatesh Perumal · Accepted solution]

suggest to go through these docs:

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-4000-series-switches/29805-175.html

http://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/25885-pmtud-ipfrag.html

https://supportforums.cisco.com/discussion/11399941/does-jumbo-frames-ever-get-fragmented

http://www.networkworld.com/article/2224722/cisco-subnet/jumbo-frames.html

[pdub206]

Thank you, that pretty much answered all my questions.