Generic Routing Encapsulation (GRE)

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Generic routing encapsulation (GRE) is a tunneling protocol allowing the transmission of encapsulated frames or packets from different types of network protocol over an IP network.

• works at layer 3
• GRE packet encapsulate an IP packet as its payload
  • or other Network layer protocol
  • outer GRE packet
    • is assigned protocol number 47
    • has its own IP source and header address fields
• GRE packet is then encapsulated in a layer 2 frame
  • for transmission to the next hop router
• Each intermediate router inspects only the outer GRE header to determine the forwarding destination
• final receiving router
  • de-encapsulates the GRE packet to extract the inner IP payload
  • forwards that inner packet to its destination
• does not have any mechanisms for authenticating users or devices
  • often used with other protocols in a VPN solution