Question 87 - 300-730 discussion

DMVPN stands for Dynamic Multipoint VPN, which is a technology that allows routers to dynamicallyform VPN tunnels with each other without requiring a pre-configured static crypto map. DMVPN usesMultipoint GRE (mGRE) interfaces and Next Hop Resolution Protocol (NHRP) to establish directconnections between routers. DMVPN has three phases of operation, each with different features andbenefits.DMVPN Phase 1 is the basic configuration, where all spokes are configured with a single mGRE interfacethat points to the hub as the NHRP server. The spokes can only communicate with the hub, not witheach other. All traffic must go through the hub, which creates a bottleneck and increases latency.DMVPN Phase 2 improves on Phase 1 by allowing spoke-to-spoke communication without going throughthe hub. This is achieved by using NHRP to dynamically resolve the IP address of the destination spokeand create a direct GRE tunnel between the spokes. However, this still requires the use of a dynamicrouting protocol to advertise routes between the spokes, which adds overhead and complexity.DMVPN Phase 3 further enhances Phase 2 by enabling spoke-to-spoke communication without requiringa dynamic routing protocol. This is done by using NHRP shortcut switching and NHRP redirect messages.When a spoke wants to send traffic to another spoke, it sends an NHRP resolution request to the hub,which responds with an NHRP redirect message containing the IP address of the destination spoke. The source spoke then creates a direct GRE tunnel with the destination spoke and switches the traffic to thenew tunnel. The hub also sends an NHRP resolution reply to the destination spoke, informing it of thesource spoke's IP address. The destination spoke then creates a direct GRE tunnel with the source spokeand switches the traffic to the new tunnel. This way, the spokes can communicate directly without usinga dynamic routing protocol or going through the hub