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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There you go, GRE in a nutshell! 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.

There is nothing to say that the two routers are running a routing protocol between them, so they could have entirely different ideas of which way to send a packet. R 1 bit Indicates the Routing Present bit.

Generic Routing Encapsulation

Retrieved May 14, Sign up or log in Sign up using Google. In addition, the receiver must ignore this field in received packets. The Reserved1 field is reserved for future use, and if present, must be transmitted as zero.


Protocol Type 16 bits Contains the type of the passenger protocol. Thanks for the answer. GRE 47 Header checksum: A receiver must discard a packet where any of bits are non-zero, unless that receiver implements RFC Indicates the Strict Source Route bit. Retrieved from ” https: Tfc, there are a few things that can be a little rough to get the hang of at first. Home Questions Tags Users Unanswered.

Indicates the Checksum Present bit. If the number of encapsulations is greater than 3, the packet is discarded. 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.

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Ver 3 bits Contains the version number. Care should be taken when forwarding such a packet, since if the destination address of the payload packet is the encapsulator of the packet i. The Reserved1 field is present only when the Checksum field is present that is, Checksum Present bit is set to one. This field is used to prevent the packet from being encapsulated all the time.

Indicates the Key field. Indicates the Sequence Number Present bit.


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The Recursion field increases by one after each encapsulation. One or more could be using static routes or PBR. There is nothing to say that the two routing tables are in sync. Sequence Number 32 bits The Sequence Number field contains an unsigned 32 bit integer which is inserted by the encapsulator. Post was not sent – check your email addresses! Router B could have a route to 1. Unsourced material may be challenged and removed.

Generic Routing Encapsulation – Wikipedia

This article needs additional citations for verification. Currently, it must be set to 0. By using this site, you agree to the Terms of Use and Privacy Policy.

The Routing field is optional and is present only if the Routing Present bit is set to 1. Cisco’s documentation seems to demostrate that grd use this exact behavior to make a keepalive work.