Canonical URL: ; File formats: Plain Text PDF; Status: PROPOSED STANDARD; Updated by: RFC ; Authors: D. Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside. If the de-encapsulating router has a route for the un-encapsulated destination address going back to the encapsulating router, you will get a.
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The Reserved1 field is present only when the Checksum field is present that is, Checksum Present bit is set to one.
Not set Fragment offset: This is an entirely plausible scenario if the rule is not followed. From Wikipedia, the free encyclopedia. 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.
This field is present if the Routing Present or the Grd Present bit is set to 1, and contains valid information only if the Routing Present bit is set to 1. Reserved1 16 bits The Reserved1 field is reserved for future use, and if present, must be transmitted as zero.
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If the number of encapsulations is greater than 3, the packet is discarded. Recur 3 bits Indicates the Recursion Control field. Contains the version number.
The tunnel source can be a physical interface or an IP address just keep in mind the tunnel source needs to be local on the router. I think I understand what you are saying, but could you provide some sample IP addresses and routes to demonstrate the concept? Based on the principles of protocol rffc in OSI, protocol encapsulation, not specifically GRE, breaks the layering order.
Currently, it must be set to 0. Globally unique address factory default Source: Sequence Number 32 bits The Sequence Number field contains an unsigned 32 bit integer tfc is inserted by the encapsulator.
Such as allowing multicast traffic to traverse the tunnel providing us scalability with the use vre a routing protocol. The offset field indicates the octet offset from the start of the Routing field to the first octet of the active Source Route Entry to be examined.
Generic Routing Encapsulation
This field contains the number of encapsulations of GRE packets. Occhiogrosso February 6, at 7: Indicates the Recursion Control field.
One or more could be using static routes or PBR. It may be viewed as a separator between two different protocol stacks, one acting as a carrier for another. However, there are a few things that can be a little rough to get the hang of at first.
You will want to make sure the tunnel source and tunnel destination configured have connectivity to each other since it is between these addresses the GRE tunnel will run over. Indicates the Sequence Number Present bit. After receiving a certain network layer protocol such as IPX packet that needs to be encapsulated and routed, the system adds a GRE header to the packet, and then encapsulates the packet into another protocol such as IP.
The Recursion field increases by one after each encapsulation. There you go, GRE in a nutshell! GRE 0 Protocol Type: Offset 16 bits The offset field indicates the octet offset from the start of the Routing field to the first octet of the active Source Route Entry to be examined.