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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It would seem that the original tunnel endpoint sending would have to have a messed up route to not recognize itself as the destination and rc the packet over the tunnel. GRE 47 Header checksum: Sign up using Facebook. Retrieved May 14, This page was last edited on 28 Novemberat October Learn how and when to remove this template message. By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

There is nothing to say that rfd two routing tables are in sync.

Globally unique address factory default Source: Contains the checksum of the GRE header and the payload. Indicates the reserved field. 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. This field is present if the Routing Present or the Checksum Present bit is set to 1, and contains valid information only if the Routing Present bit is set to 1.

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. It provides us these features because of how it interacts with the IP packet, some people refer to GRE as a layer 3. This is an entirely plausible scenario if the rule is not followed.

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Generic Routing Encapsulation – Wikipedia

Each SRE has 2874 form: Key 32 bits Indicates the Key field. Pages using RFC magic links Articles needing additional references from October All articles needing additional references.

Grre is used by the receiver to authenticate the received packet. Retrieved from ” https: Sequence Number 32 bits The Sequence Number field contains an unsigned 32 bit integer which is inserted by the encapsulator. 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.

Cisco’s documentation seems to demostrate that they use 284 exact behavior to make a keepalive work. 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. Recur 3 bits Indicates the Recursion Control field. My journey to CCIE! The rule says Router B needs to drop the packets rather than sending them back the direction from which they came.

K 1 bit Indicates the Key Present bit.

The tunnel source can be a physical interface or an IP address just keep in bre the tunnel source needs to be local on the router. Sign up or log in Sign up using Google. Locally administered address rrc is NOT the factory default Source: Indicates the Checksum Present bit. If this bit is set to 1, then the Checksum field is present in the GRE header; if this is set to 0, then the GRE header does not contain the Checksum field.

From Wikipedia, the free encyclopedia. GRE 0 Protocol Type: Routing 32 bits The Routing field is optional and is present only if the Routing Present bit is set to 1. 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.

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This field contains the number of encapsulations of GRE packets. If the de-encapsulating router has a route for the un-encapsulated destination address going gr to the encapsulating router, you will get a loop that only 27784 when the TTL expires. 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.

Generic Routing Encapsulation

If this bit is set to 1, then it indicates that the Sequence Number field is present. If the number of encapsulations is greater than 3, the packet is discarded. A receiver must discard a packet where any of bits are non-zero, unless that receiver implements RFC It may be used by the receiver to establish the order in which packets have grd transmitted from the encapsulator to the receiver. In this case, the packet MUST be discarded.

S 1 bit Indicates the Sequence Number Present bit. Globally unique address factory default Type: False] [Protocols in frame: Locally administered address this is NOT the factory default Type: Home Questions Tags Users Unanswered.

The tunnel destination is going to the address of the remote router you are terminating the tunnel to.

RFC – Generic Routing Encapsulation (GRE)

Feb 25, There you go, GRE in a nutshell! Views Read Edit View history.

Generic Routing Encapsulation over IPv4 networks.

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