[143444] in North American Network Operators' Group
Re: I'm missing 2 bytes (GRE implementation)
daemon@ATHENA.MIT.EDU (William Herrin)
Wed Aug 10 13:26:56 2011
In-Reply-To: <20110810163643.GA5409@srv03.cluenet.de>
From: William Herrin <bill@herrin.us>
Date: Wed, 10 Aug 2011 13:26:02 -0400
To: nanog@nanog.org
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
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
--=20
William D. Herrin ................ herrin@dirtside.com=A0 bill@herrin.us
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