On Wed, Aug 10, 2011 at 12:36 PM, Daniel Roesen <dr@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 ................ herrin@dirtside.com bill@herrin.us 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004