From: CCIEin2006 (ciscocciein2006@gmail.com)
Date: Fri Nov 11 2005 - 11:20:18 GMT-3
Thank you.
In regards to question 3 - I noticed from Cisco documentation that the
tunnel source and destination addresses are actually the IP address of the
physical interfaces and NOT the IP address under interface tunnel 0. I guess
the tunnel interface IP is simply a placeholder in the routing table? It
doesn't actually appear in the packet does it?
On 11/11/05, Jaycee Cockburn - BCX SS <Jaycee.Cockburn@bcx.co.za> wrote:
>
> Hi All,
> May I be nasty...rfc2784...
> http://www.rfc-editor.org/rfc/rfc2784.txt or
> http://www.javvin.com/protocolGRE.html
>
> Or
>
> A GRE encapsulated packet has the form:
>
> ---------------------------------
> | |
> | Delivery Header |
> | |
> ---------------------------------
> | |
> | GRE Header |
> | |
> ---------------------------------
> | |
> | Payload packet |
> | |
> ---------------------------------
>
> 1. The IP address of the tunnel is in the payload part
>
> 2. No, see rfc
>
> 3. I think if you read the rfc you'll understand, but how I understand
> is the router determines it must route the packet out tunnel 0, so it
> generates the packet, sees that it must go out a GRE interface, so adds
> the gre header, and then adds the normal ip header of the interface used
> as the tunnel source with a destination of the tunnel destination...
>
> Hope this makes sense...
>
> Cheers
> JC
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> CCIEin2006
> Sent: 11 November 2005 03:45 PM
> To: ccielab@groupstudy.com
> Subject: Re: GRE packet structure and order of operations
>
> Anyone want to take a stab at this?
>
> On 11/10/05, CCIEin2006 <ciscocciein2006@gmail.com> wrote:
> >
> > Hi folks,
> > Can someone kindly provide some clarification on how GRE works:
> > 1. Where does the IP address of the Tunnel interface appear in the
> > packet?
> > 2. Anyone have a good link showing a GRE packet with all the source
> > and destination IP addresses (in the encapsulated IP packet, GRE
> > header, and transport IP header?) 3. If possible explain step by step
> > the order of operations i.e step 1 the packet is routed to the tunnel
> > interface, step 2 the encapsulated packet is routed out the physical
> > interface with the source IP changed to the tunnel source IP, etc...
> > Thanks!
>
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