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 Sequence Number field contains an unsigned 32 bit integer which is inserted by the encapsulator. False] [Frame is ignored: The Recursion field increases by one after each encapsulation.

Locally administered address this is NOT the factory default Source: Globally unique address factory default Type: These bits must be sent as zero and must be ignored on receipt. The Reserved1 field is present only when the Checksum field is present that is, Checksum Present bit is set to one. The Reserved1 field is rrfc for future use, and if present, must be transmitted as zero.

Generic Routing Encapsulation – Wikipedia

In this case, the packet MUST be discarded. The rule says Router B needs to drop the packets rather than sending them back the direction from which they came.

Router B could have a route to 1. Posted in Packet Analysis. However, there are a few things that can be a little rough to get the hang of at first.

Retrieved May 14, One or more could be using static routes or PBR. If the number of encapsulations is greater than 3, the packet is discarded. If the rule didn’t exist, the packets would just bounce back and forth until the TTL expired. Indicates the Key Present bit. This article needs additional citations for verification. A receiver must discard a packet where any of bits are non-zero, unless that receiver implements RFC 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.


Indicates the Key field. Bits are reserved for future use. S 1 bit Indicates the Sequence Number Present bit. Indicates the Routing Present bit.

Unsourced material may be challenged and removed. Jun 21, Post Your Answer Discard By clicking “Post Your Answer”, you acknowledge that you have read our updated terms of serviceprivacy policy and cookie policyand that your continued use of 284 website is subject to these policies. Sign up using Email and Password. Please help improve this article by adding citations to reliable sources. 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.

Contains the checksum of the GRE header and the payload. Ver 3 bits Contains the version number. False] [Protocols in frame: It may be viewed as a separator between two different protocol stacks, one acting as a carrier for another.

Information on RFC ยป RFC Editor

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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And for a closer at the fields within the GRE encapsulation. K 1 bit Indicates the Key Present bit. Indicates the Checksum Present bit.

Post as a guest Name. Then, the IP protocol is responsible for forwarding the packet. Feb 25, Reserved1 16 bits The Reserved1 field is reserved for future use, and if present, must be transmitted as zero. Defined in RFCno anomaly occurs when the values of the Recursion field on the sender and receiver are ge.

Generic Routing Encapsulation

Thanks for the answer. 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.

Defined in RFCthe default value is 0. 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.

If the de-encapsulating router has a route for the un-encapsulated destination address going back to the encapsulating router, you will get a loop that only expires when the TTL expires. Not set Fragment offset: GRE 0 Protocol Type: This must be set to 0. Contains the version number. 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 forward the packet over the tunnel.