4.1.c Implement and troubleshoot encapsulation

4.1.c [ii] Dynamic GRE

NHRP is used similarly to the Address Resolution Protocol (ARP) on Ethernet, it provides the ability to map a tunnel IP address with a logical Non-Broadcast Multi-Access (NBMA) IP address; this allows multipoint (mGRE) to have dynamically set up tunnels without having to explicitly configure a mapping entry between each potential next-hop destination. Dynamic GRE is configured using the NHRP based address resolution for mGRE. Multipoint GRE can be used both at hub and at spokes. Dynamic GRE or Dynamic Multipoint VPNs (DMVPNs) can be configured with or without IPSec.

The “% TUN-5-RECURDOWN: Tunnel0 temporarily disabled” is caused due to recursive routing.The error message means that the GRE tunnel router has discovered a recursive routing problem. Tunnel interface status depends on the IP reachability to the tunnel destination. When the router detects a recursive routing failure for the tunnel destination, it shuts the tunnel interface down for a few minutes so that the situation causing the problem can resolve itself as routing protocols converge . If the problem is caused by misconfiguration, the link can oscillate indefinitely . Another symptom of this problem is continuously flapping Enhanced Interior Gateway Routing Protocol (EIGRP), Open Shortest Path First (OSPF), or Border Gateway Protocol (BGP) neighbors , when the neighbors are over a GRE tunnel.

This condition is usually due to one of these causes:

● A misconfiguration that causes the router to try to route to the tunnel destination address using the tunnel interface itself (recursive routing)

● A temporary instability caused by route flapping elsewhere in the network

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4465-4470).  . Kindle Edition.