Everything from checksums, keys, and sequence numbers is optional. The only required fields IIRC amount to 2 bytes of overhead. Sounds like they both interpret what should be included in the GRE header slightly differently. Stefan Fouant GPG Key ID: 0xB4C956EC Sent from my HTC EVO. ----- Reply message ----- From: "Franck Martin" <fmartin@linkedin.com> Date: Tue, Aug 9, 2011 5:57 pm Subject: I'm missing 2 bytes (GRE implementation) To: "nanog@nanog.org" <nanog@nanog.org> I'm using a GRE IPv4 tunnel between a cisco and linux machines I did some packet capture, and saw that my MTU was 1418, but the cisco was sending TCP packet with a MSS of 1380. This created a bunch of issues. When I told the cisco box to use a MSS of 1378 everything starting to work fine. So why Cisco is off by 2 Bytes? Does the GRE implementation on Linux uses 2 extra bytes compared to Cisco (or vice versa)?