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GRE tunnel

What is GRE tunnel ? please explain
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mhho
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mhho
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Bob LambersonSoftware EngineerCommented:
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mikeleebrlaCommented:
first off there isn't any such thing as a GRE tunnel.  GRE is a protocol just as TCP/IP is a protocol, IE a set of rules, IE a standard.  GRE is associated w/ PPTP VPNs (often called VPN tunnels).  For a PPTP VPN to work you have to allow the GRE protocol though your firewall.  The GRE protocol is often called 'protocol 47' but don't get this confused with TCP/IP port 47.  Often times when people are trying to allow PPTP VPNs through their firewall they allow TCP/IP port 47 though but that is 100% useless and the VPN will not work.  You need to allow the GRE protocol through instead.

http://support.microsoft.com/?kbid=241251
http://www.cisco.com/warp/public/110/pix_pptp.html
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Bob LambersonSoftware EngineerCommented:
Here is a link to a Cisco site that describes "GRE tunneling" - to say there is no such thing is misleading.

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_tech_note09186a0080093f1f.shtml#intro
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