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Attention!From 1st January 2015 the European Union changes the tax regulations.Please choose appropriate tax region below. As an account security measure, you will need to grant access to this browser by entering the special code. For more information on document conventions, refer to the Cisco Technical Tips Conventions. Normally, a GRE Tunnel interface comes up as soon as it is configured and it stays up as long as there is a valid tunnel source address or interface which is up.
These three rules (missing route, interface down and mis-routed tunnel destination) are problems local to the router at the tunnel endpoints and do not cover problems in the intervening network.
Keepalive messages are sent by one network device via a physical or virtual circuit to inform another network device that the circuit between them still functions.
This is sample output from the debug serial interface command for an HDLC connection when keepalives are enabled. The GRE tunnel keepalive mechanism is slightly different than for Ethernet or serial interfaces. Another attribute of GRE tunnel keepalives is that the keepalive timers on each side are independent and do not have to match. A GRE tunnel is a logical interface on a Cisco router that provides a way to encapsulate passenger packets inside a transport protocol. These packets illustrate the IP tunneling concepts where GRE is the encapsulation protocol and IP is the transport protocol. In order to better understand how the tunnel keepalive mechanism works, consider this example tunnel topology and configuration. This is an example of a keepalive packet that originates from Router A and is destined for Router B.
This table shows the configuration of Router A and Router B with tunnel keepalive configured on both routers.
In the case when Router B is unreachable, Router A continues to construct and send keepalive packets as well as normal traffic. GRE tunnels are sometimes combined with IPSec because IPSec does not support IP multicast packets. This diagram shows what happens when you configure tunnel protection on the GRE tunnel interface. The IPSec crypto-map is tied to the physical interface and is checked as packets are forwarded out the physical interface.
Tunnel protection ties the encryption functionality to the GRE tunnel and is checked after the packet is GRE encapsulated but before the packet is handed to the physical interface. A problem occurs if a crypto map is used (configured on the physical interface) on one side of a GRE IPSec Tunnel and Tunnel Protection is configured on the other side. You can avoid this problem and there can be a benefit if GRE keepalive is configured on the router where tunnel protection is configured.
If the hub uses dynamic crypto-maps and the spoke router uses tunnel protection, then you can configure GRE keepalives on the spoke router so you can to trigger a backup interface to dial the Hub if the tunnel interface shuts down on the spoke. Although the GRE tunnel interface on the hub remains up, the routing neighbor and the routes through the tunnel are lost and the alternate route can be established. If both the routers are configured with crypto-maps, the tunnel keepalives can get through in both directions and the GRE tunnel interfaces can shut down in either or both directions, and trigger a backup connection to be made.
If both routers are configured with tunnel protection then GRE tunnel keeaplives cannot be used in either direction. The GRE tunnel keepalives are not supported in conjunction with the tunnel protection ipsec profile command. This means that each tunnel end-point does not keep any information about the state or availability of the remote tunnel end-point. For example, these rules do not cover the case in which the GRE tunneled packets are successfully forwarded, but are lost before they reach the other end of the tunnel. With this change, the tunnel interface dynamically shuts down if the keepalives fail for a certain period of time.

The keepalive interval is the period of time between each keepalive message that is sent by a network device.
Since there are many possible neighbors on the Ethernet, the keepalive is not designed to determine if the path to any one particular neighbor on the wire is available. Serial keepalives are sent back and forth between two routers and the keepalives are acknowledged. When the difference in the values in the myseq and mineseen fields exceeds 3 on Router 2, the line goes down and the interface is reset.
It gives the ability for one side to originate and receive keepalive packets to and from a remote router even if the remote router does not support GRE keepalives. The problem with the configuration of keepalives only on one side of the tunnel is that only the router that has keepalives configured marks its tunnel interface as down if the keepalive timer expires. It is an architecture designed to provide the services to implement a point-to-point encapsulation scheme. The passenger protocol is also IP (although it can be another protocol like Decnet, IPX or Appletalk). The physical interfaces of Router A and Router B are S1 and S2 respectively and the tunnel interfaces are called Tu0.
The keepalive response that Router B returns to Router A is already inside the Inner IP Header. It then sends that packet out its tunnel interface, which results in the encapsulation of the packet with the outer IP header and a GRE header with PT = IP. This prevents dynamic routing protocols from running successfully over an IPSec VPN network. The packets come into the router encrypted via the tunnel interface and are de-crypted and de-capsulated before they continue towards their destination as clear-text.
On the spoke, the fact that the tunnel interface went down can trigger it to bring up a dialer interface and call back to the hub (or another router at the hub), then establish a new connection. In this case you must use the routing protocol or other mechanism, such as the Service Assurance Agent, to discover that the tunnel is not functional. Those working on the program have managed to source hundreds of connections in locations across the world that are all available free of charge and give peace of mind that you are working under a blanket of privacy.Unlike other programs, SoftEther VPN, which has been open source since January 2015, creates a virtual network adapter on your Windows PC and connects to the internet through this thus giving yet another layer of privacy. A consequence of this is that the local tunnel end-point router does not have the ability to bring the line protocol of the GRE Tunnel interface down if the remote end of the tunnel is unreachable.
In order to better understand how GRE tunnel keepalives work, these sections discuss some other common keepalive mechanisms. The keepalive retries is the number of times that the device continues to send keepalive packets without response before the interface is brought down. It is only designed to check that the local system has read and write access to the Ethernet wire itself.
With the use of sequence numbers, each router keeps track of the keepalive packets sent and acknowledged. Since GRE is a packet tunneling mechanism for tunneling IP inside IP, a GRE IP tunnel packet can be built inside another GRE IP tunnel packet.
The GRE tunnel interface on the other side, where keepalives are not configured, remains up even if the other side of the tunnel is down.
Router B simply decapsulates the keepalive packet and sends it back out the physical interface (S2). If that condition is not true, then the next time Router A attempts to send a keepalive to Router B, the line protocol is brought down. On the reception of a keepalive response, with the implication that the tunnel endpoint is again reachable, the tunnel keepalive counter is reset to 0 and the line protocol on the tunnel comes up. Since GRE tunnels do support IP multicast, a dynamic routing protocol can be run over a GRE tunnel.
Both methods specify that IPSec encryption is performed after the addition of the GRE encapsulation.
Once the packets are de-crypted and de-encapsulated, they continue on to their IP destination as clear-text. The ability to mark an interface as down when the remote end of the link is not available is used in order to remove any routes (specifically static routes) in the routing table that use that interface as the outbound interface.

This means that a static route or PBR forwarding of packets via the GRE tunnel interface remains in effect even though the GRE tunnel packets do not reach the other end of the tunnel.
Keepalives on the GRE tunnel interface are used in order to solve this issue in the same way as keepalives are used on physical interfaces. The router produces an Ethernet packet with itself as the source and destination MAC-address and a special Ethernet type code of 0x9000. In this way, the remote routers look at each others keepalives and track if the keepalives they send are received.
For GRE keepalives, the sender pre-builds the keepalive response packet inside the original keepalive request packet so that the remote end only needs to do standard GRE decapsulation of the outer GRE IP header and then forward the inner IP GRE packet. The tunnel can become a black-hole for packets directed into the tunnel from the side that did not have keepalives configured. With the assumption that there is a way to reach the far end tunnel endpoint and the tunnel line protocol is not down due to other reasons, the packet arrives on Router B. When a crypto-map is used, it is applied to the outbound physical interface(s) for the GRE tunnel packets.
Tunnel protection is configured on the hub router in order to reduce the size of the configuration and a static crypto map is used on each spoke.
Specifically, if the line protocol for an interface is changed to down, then any static routes that point out that interface are removed from the routing table. The Ethernet hardware sends this packet onto the Ethernet wire and then immediately receives this packet back again. This mechanism causes the keepalive response to forward out the physical interface rather than the tunnel interface. In a large hub-and-spoke GRE tunnel network, it might be appropriate to only configure GRE keepalives on the spoke side and not on the hub side.
It is then matched against Tunnel 0, becomes decapsulated, and is forwarded to the destination IP which is the tunnel source IP address on Router A. When you configure GRE tunnel keepalive on the spoke in this scenario, the keepalive fails. Pull up the list underneath the simulated country to bring up a list of countries that you can change your location to and there is a large number available for free and others come as part of a premium subscription.Once you've chosen a country click it and wait for CyberGhost VPN to do its work to spoof your location. This allows for the installation of an alternate (floating) static route or for Policy Based Routing (PBR) to select an alternate next-hop or interface. This checks the sending and receiving hardware on the Ethernet adapter and the basic integrity of the wire. This is because it is often more important for the spoke to discover that the hub is unreachable and therefore switch to a backup path (Dial Backup for example).
Upon arrival on Router A, the packet becomes decapsulated and the check of the PT results in 0. This is because the return keepalive from the HUB traverses the physical interface which has no crypto-map configured. You can go even further than that by changing the simulated IP address, if the location has more than one option, and this will enable you to pick a server that has less subscribers.To make your new location look even more genuine, CyberGhost VPN can be set up to change the browser language and it can remove the browser or operating system version.
Therefore, the return keepalive does not get encrypted and the receiving router (which originally sent the keepalive packet) drops the keepalive response because it was not IPSec protected (encrypted). KeyScrambler Personal is a stripped down encryption service that works to do one thing, which is to encrypt everything you type into a browser.KeyScrambler Personal is turned on automatically for every browser on your system and it will pop up when you start typing to show what the output has been encrypted as. This is done through a disk cleanup facility that has three options ranging from basic (empties recycle bin and deletes newsgroups) to thorough (the same as basic as well as deletes cookies, cleans up all users, deletes favourite places and bookmarks from all files, resets scores of standard games and fully erases files). The latter shouldn't be taken lightly as it erases your PC back to a similar states to when it was bought.When CleanUp has done its stuff it should leave you with much needed space on your PC and all this comes completely free of charge.

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