Good point. Should permit compatibility with Cisco, Linux, Vyatta and others...
I hope that Keepalive option will be present, so that we can have a statefull tunel for static routes.
We need this capability for a link to a provider where we cannot use BGP. He has only support for Cisco GRE, so there is no other possibility than using GRE + Keepalive.
According to Cisco about Keepalive option :
"GRE tunnels are designed to be completely stateless. This means that each tunnel end-point does not keep any information about the state or availability of the remote tunnel end-point. 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. 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. 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. 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.
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. The tunnel destination IP address must also be routable. This is true even if the other side of the tunnel has not been configured. 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.
Before GRE keepalives were implemented, there were only three reasons for a GRE tunnel to shut down:
There is no route to the tunnel destination address.
The interface that anchors the tunnel source is down.
The route to the tunnel destination address is through the tunnel itself.
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. 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. This causes data packets that go through the GRE tunnel to be "black holed", even though an alternate route that uses PBR or a floating static route via another interface is potentially available. 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.
With Cisco IOS® Software Release 12.2(8)T, it is possible to configure keepalives on a point-to-point GRE tunnel interface. With this change, the tunnel interface dynamically shuts down if the keepalives fail for a certain period of time. In order to better understand how GRE tunnel keepalives work, these sections discuss some other common keepalive mechanisms. "