If the length of a packet by attaching GRE-IP header results in bigger size than router MTU, what happen on More Fragment Flag and Frag Offset field of that packet?
GRE Fragmentation – More Fragment Flag and Frag Offset Field Explained
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If the length of a packet by attaching GRE-IP header results in bigger size than router MTU, what happen on More Fragment Flag and Frag Offset field of that packet?
Best Answer
Often, IP packets will need to be fragmented before encapsulation by GRE. The tunnel interface will have a smaller MTU than the physical router interface (physical interface MTU - GRE packet overhead). The tunnel interface is treated as a real interface, and the IP packets entering the tunnel will need to be fragmented before GRE encapsulation.
See RFC 7588, A Widely Deployed Solution to the Generic Routing Encapsulation (GRE) Fragmentation Problem:
And:
RFC 791, INTERNET PROTOCOL, has a complete discussion on fragmentation, including examples: