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Concept
Generic Routing Encapsulation
Summary
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 or point-to-multipoint links over an Internet Protocol network.
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.
Based on the principles of protocol layering in OSI, protocol encapsulation, not specifically GRE, breaks the layering order. It may be viewed as a separator between two different protocol stacks, one acting as a carrier for another.
GRE packets that are encapsulated within IP directly, use IP protocol type 47 in the IPv4 header's Protocol field or the IPv6 header's Next Header field.
For performance reasons, GRE can also be encapsulated in UDP packets. Better throughput may be achieved by using Equal-cost multi-path routing.
The extended version of the GRE packet header is represented below:
{| class="wikitable" style="text-align: center"
|+Extended GRE header format
|-
!style="border-bottom:none; border-right:none;"|Offsets
!style="border-left:none;"|Octet
!colspan="8"|0
!colspan="8"|1
!colspan="8"|2
!colspan="8"|3
|-
!style="border-top: none"|Octet
!Bit
!style="width:2.75%;"|0
!style="width:2.75%;"|1
!style="width:2.75%;"|2
!style="width:2.75%;"|3
!style="width:2.75%;"|4
!style="width:2.75%;"|5
!style="width:2.75%;"|6
!style="width:2.75%;"|7
!style="width:2.75%;"|8
!style="width:2.75%;"|9
!style="width:2.75%;"|10
!style="width:2.75%;"|11
!style="width:2.75%;"|12
!style="width:2.75%;"|13
!style="width:2.75%;"|14
!style="width:2.75%;"|15
!style="width:2.75%;"|16
!style="width:2.75%;"|17
!style="width:2.75%;"|18
!style="width:2.75%;"|19
!style="width:2.
Official source
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