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

[William Herrin <bill () herrin us>]

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




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