On Wed, Aug 10, 2011 at 12:36 PM, Daniel Roesen <d...@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: http://en.wikipedia.org/wiki/Generic_Routing_Encapsulation#Packet_header 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. Regards, Bill Herrin -- William D. Herrin ................ her...@dirtside.com b...@herrin.us 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004
