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…

Question

Options

• ABecause dynamic routing is not enabled
• BBecause the tunnel cannot reach its tunnel destination
• CBecause the best path to the tunnel destination is through the tunnel itself
• DBecause the router cannot recursively identify its egress forwarding interface.

• A
  2% (1)
• B
  5% (2)
• C
  85% (35)
• D
  7% (3)

Explanation

GRE Recursive Routing Explained

Option C is correct because recursive routing occurs when the router's routing table directs tunnel destination traffic back through the tunnel itself, creating an infinite loop - the router disables the tunnel to prevent this. This is a classic misconfiguration where the tunnel destination IP address is learned via the tunnel interface rather than through a physical (underlay) interface. The %TUN-RECURDOWN message is Cisco's protection mechanism against this loop condition.

Why the distractors are wrong:

• A is incorrect because dynamic routing is not required for a GRE tunnel to function; static routes work fine, and the error is specifically about routing loops, not missing routing protocols.
• B is incorrect because the tunnel can technically reach its destination - the problem is it's trying to reach it via itself, not that the path is missing entirely.
• D is incorrect because the router can identify an egress interface; it incorrectly identifies the tunnel interface itself as that egress, which is the root of the problem.

Memory Tip: Think of recursive routing as a snake eating its own tail - the tunnel tries to use itself to reach its own endpoint. The fix is always to ensure the tunnel destination IP is reachable via a physical interface route, not through the tunnel.

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