GRE Keepalives

2 years back When I was entered in service provider domain, the first I learned how to create GRE tunnel and keepalive is the mandid to get the tunnnel work properly. During googling I found a good document of cisco which is exlicitly defining the keepalive working.

Two routers R1 -> R2 are connected back to back.

When you enable keepalives on the tunnel endpoint of Router A, the router at every interval constructs the inner IP header. At the end of the header, the router also appends a GRE header with a Protocol Type (PT) of 0, and no other payload. The router then sends that packet through the tunnel, which results in its encapsulation with the outer IP header, and a GRE header with the PT of IP. The tunnel keepalive counter increments by one. If 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. It is then matched against Tunnel 0, is
decapsulated, and forwarded to the destination IP, which is the tunnel source, Router A. Upon arrival on Router A, the packet is again decapsulated, and the PT is checked. If the result of the PT check is 0, it signifies that this is a keepalive packet. In such a case, the tunnel keepalive counter is reset to 0, and the packet is discarded. In case Router B is unreachable, Router A continues to construct and send the keepalive packets along with
normal traffic. If the line protocol is down, the keepalives do not come back to Router A. Therefore, the keepalive counter continues to increase. The tunnel line protocol stays up only as long as the tunnel keepalive counter remains zero, or less than a configured value. If that condition is not true, the next time you attempt to send a keepalive to Router B, the line protocol is brought down, as soon as the keepalive counter reaches the configured keepalive value. In the up/down state, the tunnel does not forward or process any traffic apart from the keepalive packets. For this to work for keepalive packets only, the tunnel must be forward-and-receive friendly. So the tunnel lookup algorithm must be successful in all cases, and must discard only the data packets if the line protocol is down. When a keepalive packet is received, it implies that the tunnel endpoint is again reachable. The tunnel keepalive counter is then reset to 0, and the line protocol comes back up.


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