The following system log message is presented after a network administrator configures a GRE tunnel: %TUN-RECURDOWN Interface Tunnel 0 temporarily disabled due to recursive routing Why is Tunnel 0 disabled?
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A. B. C. D.C.
The system log message "%TUN-RECURDOWN Interface Tunnel 0 temporarily disabled due to recursive routing" indicates that Tunnel 0 has been temporarily disabled due to recursive routing. Recursive routing occurs when a router receives a packet that needs to be forwarded out of the same interface that it was received on. This can create an endless loop that can cause network congestion and ultimately a network outage.
In the case of the GRE tunnel, the most likely cause of the recursive routing is that the best path to the tunnel destination is through the tunnel itself. This can happen if the routing protocol used by the router does not differentiate between the tunnel interface and the physical interface. When the router receives a packet destined for the tunnel, it tries to forward it through the tunnel, but because the tunnel is the best path to the tunnel destination, the packet gets looped back into the tunnel interface, creating a recursive loop.
To prevent this from happening, the router temporarily disables the Tunnel 0 interface to prevent further traffic from entering the tunnel until the issue is resolved. To fix the issue, the network administrator can either modify the routing protocol configuration to prevent recursive routing or modify the static routes or routing metrics to ensure that the tunnel is not the best path to the tunnel destination.