Help Learning GRE

Unanswered Question
Dec 29th, 2008

I'm just learning GRE tunneling right now. I'm a little confused about how a GRE tunnel is supposed to work to get a packet to the destination. Does IP encapsulate a GRE tunnel with the IP address of the destination? If so, how does this work. It seems that a packet is already handed down from the Network to the Data Link layer, and there the tunnel is added, which is where GRE is added. How then if the packet is already down at the data link layer does IP encapsulate it? And if IP doesn't, could someone explain it to me so that I better understand how GRE works from end to end. Thank you,

J.D.

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lejoe.thomas Mon, 12/29/2008 - 22:40

Hi Edmond,

With GRE,

GRE is the encapsulation protocol

Lets say a packet (eg :IPV4 packet) needs to be delivered, which is often referred to as a payload. GRE encapsulates this payload with appropriate protocol type indicating that IPV4 packet is the payload. GRE in turn is encapsulated in other protocol such as IP with tunnel destination as destination address etc.

When the packet reaches the tunnel destination, the GRE packet is decapsulated and IPV4 payload is forwarded just like a normal IPV4 packet.

Consider reading the RFCs to get a better understanding of GRE.

http://www.faqs.org/rfcs/rfc2784.html

HTH

Lejoe

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