RFC 2784: https://tools.ietf.org/html/rfc2784
https://www.juniper.net/documentation/en_US/junos/topics/topic-map/switches-interface-gre.html
| The GRE frame contains: | outer MAC header, outer IP header, GRE header, original layer 2 frame, and frame checksum (FCS) |
|---|---|
| In Layer 2 service | GRE 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 | L3 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 |