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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This page was last edited on 28 Novemberat Recur 3 bits Indicates the Recursion Control field.

By using this site, you agree to rdc Terms of Use and Privacy Policy. The first thing unlike physical interface you can declare tunnel interfaces from configuration mode and you can start at any number.

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 the website is subject to rtc policies. 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.

Indicates the Recursion Control field. And for a closer at the fields within the GRE encapsulation.


Generic Routing Encapsulation – Wikipedia

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. There is nothing to 2748 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.

Please help improve this article by adding citations to reliable sources. R 1 rvc Indicates the Routing Present bit. K 1 bit Indicates the Key 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.

Generic Routing Encapsulation over IPv4 networks.

GRE 47 Header checksum: However, there are a few things that can be a little rough to get the hang of at first. Thanks for the answer.

Generic Routing Encapsulation

This field is present if the Routing Present or the Checksum Present bit gde set to 1, and contains valid information only if the Routing Present bit is set to 1. 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. These bits must be sent as zero and must be ignored rfv receipt. Retrieved May 14, Bits are reserved for future use.


Email Required, but never shown. The Recursion field increases by one after each encapsulation. Notice you can see two sets of IP address in that packet, the first set of IP addresses Written by Stephen J. Retrieved from ” https: It may be used by the receiver to establish the order in which packets have been transmitted from the encapsulator to the receiver.

It provides us these features because of how it interacts with the IP packet, some people refer to GRE as a layer 3. Flags 5 bits Indicates the reserved field.

Each SRE has the form: If this bit is set to 1, then it indicates that the Offset gee Routing fields are present and contain valid information. S 1 bit Indicates the Sequence Number Present bit.

RFC | CCIE or Null!

Unsourced material may be challenged and removed. The rule says Router B needs to drop the packets rather than sending them back the direction from which they came. This article needs additional citations for verification. Not set Fragment offset: