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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Offset 16 bits Tre 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. S 1 bit Indicates the Sequence Number Present bit.
Indicates the Routing Present bit. GRE 47 Header checksum: Generic Routing Encapsulation over IPv4 networks.
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. 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.
Indicates the Checksum Present bit. Protocol Type 16 bits Contains the type of the passenger protocol. This article needs additional citations for verification.
This is an entirely plausible scenario if the rule is not followed. The rule says Router B needs to drop the packets yre than sending them back the direction from which they came. Globally unique address factory default Type: Such as allowing multicast traffic to traverse the tunnel providing us scalability with the use of a routing protocol.
Flags 5 bits Indicates the reserved field. It is recommended that this bit only be set to 1 if all of the Routing Information consists of Strict Source Routes.
Routing 32 bits The Routing field is optional and is present only if the Routing Present bit is set to 1. Indicates the Recursion Control field. A receiver must discard a packet where any of bits are non-zero, unless that receiver implements RFC If router B does send them back, Router A is supposed to discard them, too, based on the rule.
The tunnel destination is going to the address of the remote router you are terminating the tunnel to. These bits must be sent as zero and must be ignored on receipt.
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This must be set to 0. 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. From Wikipedia, the free encyclopedia. False] [Frame is ignored: I think I understand what you are saying, but could you provide some sample IP addresses and routes to demonstrate the concept?
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Cisco’s documentation seems to demostrate that they use this exact behavior to make a keepalive work. Key 32 bits Indicates the Key field.
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.
Thanks for the answer.
Not set Fragment offset: 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. Views Read Edit View history.
The Reserved1 field is reserved for future use, and if present, must be transmitted as zero. After receiving a certain network layer protocol such as IPX packet that needs to be encapsulated and routed, the grd adds a GRE header to the packet, and then encapsulates the packet into another protocol such as IP.
Indicates the Sequence Number Present bit. Recur 3 bits Indicates the Recursion Control field. Email Required, but never shown. Then, the IP protocol is responsible for forwarding the packet.