Re: I'm missing 2 bytes (GRE implementation)

Stefan Fouant sfouant at
Wed Aug 10 01:23:52 UTC 2011

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 at>
Date: Tue, Aug 9, 2011 5:57 pm
Subject: I'm missing 2 bytes (GRE implementation)
To: "nanog at" <nanog at>

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)?

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