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Generic Routing Encapsulation (GRE) is a way to carry one network-layer packet inside another. An outer delivery header moves the resulting packet across a network, while the GRE header identifies the type of packet being carried. GRE provides encapsulation; it does not, by itself, encrypt traffic or guarantee delivery.
What does GRE stand for?
GRE stands for Generic Routing Encapsulation. The name describes its role: GRE wraps a packet from one network-layer protocol so it can be transported within a packet of another network-layer protocol. RFC 2784 defines it as “a protocol for encapsulation of an arbitrary network layer protocol over another arbitrary network layer protocol.” RFC 2784
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How does a GRE packet work?
A GRE packet has three conceptual parts, in order:
- Delivery header: The outer protocol header carries the packet toward its destination.
- GRE header: This identifies the protocol type of the packet being encapsulated.
- Payload packet: The original network-layer packet being carried.
The outer delivery protocol transports the GRE packet; the GRE protocol-type field tells the receiving side what kind of packet is inside. RFC 2784’s base header includes a checksum-present flag, reserved bits, a version field, and the protocol type. The checksum and reserved fields are present when the checksum-present bit is set. RFC 2784
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What do the GRE key and sequence number do?
GRE can use optional extensions defined in RFC 2890. They are not necessarily present in every GRE packet. RFC 2890
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Key field
The optional four-octet Key identifies a traffic flow or context within a tunnel. Despite its name, it is not a cryptographic key and does not secure the packet; RFC 2890 explicitly says the field “is not involved in any sort or security.”
Sequence Number field
The optional four-octet Sequence Number can help a receiver determine the order in which packets were transmitted. RFC 2890 describes this as an unreliable, in-order delivery use: it does not make GRE delivery reliable or guarantee that packets arrive.
Does GRE encrypt traffic?
No. GRE encapsulates packets but does not itself provide encryption or guarantee delivery. If GRE traffic needs security protection, RFC 2784 discusses protecting it with IPsec ESP or AH. The GRE Key is not a substitute for cryptographic protection. RFC 2784 RFC 2890
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsWhat is GRE used to describe?
GRE is a protocol mechanism, not a particular router, service, or consumer product. Its core idea is packet-in-packet transport: an outer protocol delivers the GRE packet, and GRE identifies the encapsulated payload. The standards specify this behavior; they do not require a specific product to understand the definition.
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