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GRE Tunneling

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description GRE Tunneling Overview

Generic Routing Encapsulation (GRE) is a tunneling protocol used to encapsulate a network layer protocol within another. While not a VPN itself, it is often used *underneath* a VPN or tunneling solution to carry non-IP traffic or to simplify routing across complex network boundaries. It requires a separate encryption layer (like IPsec) to be secure, making it a foundational networking tool rather than a standalone VPN solution.

help GRE Tunneling FAQ

Why is GRE not considered a secure VPN by itself?

GRE encapsulates packets so they can travel through another IP network, but it does not provide encryption or strong authentication. Anyone who can inspect the path may still read the encapsulated traffic unless GRE is combined with a security layer such as IPsec.

Why combine GRE with IPsec?

GRE can carry multicast traffic and routing protocols that plain policy-based IPsec may not handle conveniently. IPsec then adds confidentiality, integrity, and peer authentication to the GRE tunnel.

What IP protocol number does GRE use?

GRE is identified as IP protocol number 47, which is not the same thing as TCP or UDP port 47. Firewalls and NAT devices must therefore support or explicitly permit the GRE protocol.

Can OSPF or other dynamic routing protocols run through GRE?

Yes, GRE's ability to carry multicast and encapsulated network-layer traffic makes it useful for protocols such as OSPF and EIGRP. This is one reason GRE-over-IPsec has traditionally been used between routed sites.

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