GRE tunnel

Posted on 2006-06-02
Last Modified: 2008-01-09
What is GRE tunnel ? please explain
Question by:mhho
    LVL 12

    Expert Comment

    LVL 25

    Expert Comment

    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.
    LVL 12

    Accepted Solution

    Here is a link to a Cisco site that describes "GRE tunneling" - to say there is no such thing is misleading.

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