Generic Routing Encapsulation (GRE) is a simple tunneling protocol that wraps one packet inside another. The original packet, which can be IPv4, IPv6 or even multicast and routing protocol traffic, becomes the payload of a new IP packet with a GRE header, using IP protocol number 47. GRE lets you build a virtual point-to-point link across a network that does not know about your addresses, such as the internet, and run routing protocols over it. GRE provides no encryption or authentication of its own, which is why it is often combined with IPsec.
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