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RFC 2784:   https://tools.ietf.org/html/rfc2784

Image Modified


https://www.juniper.net/documentation/en_US/junos/topics/topic-map/switches-interface-gre.html

The GRE frame contains:outer Router MAC header, outer IP header, GRE header, original layer 2 frame, and frame checksum (FCS)
In Layer 2 service

IP protocol 47 / GRE / L2


Code Block
titleGRE
max MTU 1514

L2 header=14 Bytes
IP header= 20 Bytes
GRE header= 4 Bytes ( or 8Bytes:  +4Bytes for options like checksum)
L2 customer packet: 1514- 38 = 1476 bytes         

Total= 38 Bytes of L2 overhead
Ratio: 42/1514 = 2.8%


In Layer 3 service

IP protocol 47 / GRE / L3


Code Block
titleL3 services
L2 header=14 Bytes
IP header= 20 Bytes
GRE header= 4 Bytes  ( or 8Bytes:  +4Bytes for options like checksum)
L3 customer Header packet: 1514 - 38 = 1476 bytes 

Total= 24 Bytes of L2 overhead
Ratio: 24/1514 = 1.6%



Also Ethernet payload / IP frame = 1500 Bytes

in GRE: payload - 24Bytes = 1500 - 24 = 1476 Bytes


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