I'd like a little clarification regarding GRE encapsulation.
From the way I have always understood it, an IP packet is encapsulated into/by a GRE packet that has a 20 byte header. But nowhere in that header exists a source IP or destination IP address field (The tunnel source and destination addresses would be the ones of interest to GRE). So, how does GRE forward the packet to the tunnel destination?
Moreover, it seems that, logically speaking, GRE sits between IP (network layer) and the data link layer. Is that correct?
I have yet to find any web link, including Cisco's, that really explains the nuts and bolts -- the mechanics -- of GRE to the degree I am describing here. Does anyone have one?
My understanding of GRE appears to be very different from yours. Yes GRE involves a 20 byte header - and that is an additional IP header (what is the length of an IP header?). In the GRE header the source address is the address specified in tunnel source and the destination address is the address specified in tunnel destination.
If you have not done so before, I suggest that you try a packet capture (wireshark or whatever you prefer) on a GRE packet.
What I think MAY be the case is that the IP datagram gets encapsulated by GRE, BUT the IP header remains "outside." So, what you have is the IP header, then the GRE header, and then the IP datagram inside the data portion of the GRE packet. Then ALL of that gets encapsulated by a L2 frame.
[EDIT] Better stated, I think GRE creates a NEW IP header with the tunnel souce and destination addresses, and THAT header's info is used to route to the tunnel endpoint. [EDIT]
This is what I think MAY be the case, although I cant seem to find any really good documentation on the nitty gritty mechanics of it.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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