I'm missing 2 bytes (GRE implementation)

William Herrin bill at herrin.us
Wed Aug 10 12:26:02 CDT 2011

On Wed, Aug 10, 2011 at 12:36 PM, Daniel Roesen <dr at cluenet.de> wrote:
> On Wed, Aug 10, 2011 at 12:57:44AM +0000, Franck Martin wrote:
>> I'm using a GRE IPv4 tunnel between a cisco and linux machines
>> So why Cisco is off by 2 Bytes?
> The only GRE options using 2 bytes are GRE checksum and offset. Haven't
> seen any of them being used by default by IOS. IOS default GRE payload
> MTU traversing an IPv4 MTU 1500 egress interface is 1476 (1500 minus 20
> octets IPv4 header, 4 octets GRE header).

Handy reference:

The GRE header length will be evenly divisible by 4. If the checksum
is present then so is the offset, and vice versa. So if you're seeing
a 2 byte (not 4 byte) difference that's coming from somewhere else.

Bill Herrin

William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004

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