How many headers are added to a packet traversing a GRE tunnel ?

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May 19th, 2006
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I need to know whether a packet if traversing a GRE tunnel Ip over Gre over IP then, is the headers added to the original packet is arranged as per the attachment doc.

Thanks in adv. I need to understand the header format in detail.



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atif.awan Fri, 05/19/2006 - 03:45
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GRE adds 24 bytes to the IP packet being encapsulated in it. The 24 bytes are composed of a 4 byte GRE header and 20 byte new IP header. Please note that the original IP packet will have a 20 byte header of its own, the 24 bytes I have mentioned are on top of that.

You should find the GRE RFC quite helpful in understanding the GRE protocol:

kaustav.gupta Fri, 05/19/2006 - 04:15
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Ok.. Then should I understand the GRE header of 4 bytes but the + 20 byte delivery IP header ,in that what is the source and destination address. Does the source IP address = Tunnel interface IP or it is the tunnel source IP and and does the same applies for the destination IP address also. The rfc does not throw light on that



atif.awan Fri, 05/19/2006 - 04:23
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In the new IP header the source IP address will be determined by the 'tunnel source' command while the destination IP address will be determined by the 'tunnel destination' command under the tunnel interface. This ensures that the resulting IP packet is able to get to the remote tunnel end-point.


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