Hello All!
There was recently a question on OSL regarding fragmentation on IPSEC and GRE tunnels. This was a shady topic to me and i decided to do some study on it. After some search, i found this incredible whitepaper from Cisco which had all the details that i was looking for. You can find the link to the document below.
http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml
I love the graphics on the whitepaper which give us a detailed picture of the fragmentation process in tunnels and the different places where the packet can get fragmented in a GRE tunnel . I must say, this is DEFINITELY one of the most informative documents that i’ve come across and i hope it helps you as much as it did to me.
Cheers,
TacACK
EDIT : I have to admit, this paper pretty lengthy and time-consuming , so i would suggest giving it about 1.5 – 2 hours of quiet time for all of it to seep in . Again, that’s just what works for me 
