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Generic routing encapsulation (GRE)

Tunneling provides a mechanism to transport packets of one protocol within another protocol. Generic routing encapsulation (GRE) is a communication protocol used to establish a direct, point-to-point connection between network nodes. Generic routing encapsulation (GRE) provides a private, secure path for transporting packets through an otherwise public network by encapsulating (or tunneling) the packets.

Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol network.

GRE encapsulates data packets and redirects them to a device that de-encapsulates them and routes them to their final destination. This allows the source and destination switches to operate as if they have a virtual point-to-point connection with each other (because the outer header applied by GRE is transparent to the encapsulated payload packet).

Packet Header 

Image result for Packet header of gre

C  

Checksum bit. Set to 1 if a checksum is present.

K
Key bit. Set to 1 if a key is present.
S
Sequence number bit. Set to 1 if a sequence number is present.
Reserved0
Reserved bits; set to 0.
Version
GRE Version number; set to 0.
Protocol Type
Indicates the ether protocol type of the encapsulated payload. (For IPv4, this would be hex 0800.)
Checksum
Present if the C bit is set; contains the checksum for the GRE header and payload.
Key
Present if the K bit is set; contains an application-specific key value.
Sequence Number
Present if the S bit is set; contains a sequence number for the GRE packet.

Uses OF GRE

  • In conjunction with PPTP to create VPNs.
  • In conjunction with IPsec VPNs to allow passing of routing information between connected networks.
  • In mobility protocols.
  • In A8/A10 interfaces to encapsulate IP data to/from Packet Control Function (PCF).
  • Linux and BSD can establish ad-hoc IP over GRE tunnels which are interoperable with Cisco equipment.
  • Distributed denial of service (DDoS) protected appliance to an unprotected endpoint.
  • Use of multiple protocols over a single-protocol backbone
  • Providing workarounds for networks with limited hops
  • Connection of non-contiguous subnetworks
  • Being less resource demanding than its alternatives (e.g. IPsec VPN)

NOTE * GRE is described in RFC 2784 (obsoletes earlier RFCs 1701 and 1702)

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