DMVPN Phase 3

DMVPN Phase 2 and Phase 3 are identical however in Phase 3, there is no need for the command "no ip next-hop-sef eigrp 10". 

To implement Phase 3, we will have to configure one additional command "ip nhrp redirect" on the hub and one additional command on the spokes ""ip nhrp shortcut". 

The idea here is that when a spoke try to send packets to another spoke, the initial packet will be forwarded to the hub. Upon receiving the packet on mGRE interface, hub will reply back with an "NHRP Redirect" packet which will indicate that the spoke is not using an optimal path and it should look for alternate way by using NHRP reosolution.

The spoke will then sends an NHRP request directly to the destination spoke address (not the hub). The destination spoke will reply abck with the IP routing prefix. When the source spoke receives this reply, it will then know the NBMA IP address of the destination.

DMVPN Phase 2

Continuing from the previous post, let's now see how phase 2 works. 

The major difference between phase 1 and phase 2 is that in phase 2, the spokes can communicate to other spokes directly by using the NHRP mapping information from the hub.

To enable the spoke to create dynamic tunnels, we will remove the tunnel destination command and change the tunnel mode to "gre multipoint".

At the moment, we can see that R2 is learning the loopback of R3 with the next hop 10.1.1.1 (hub's tunnel IP). 

DMVPN Phase 1

As shown in the diagram below, let's assume that a customer has three sites. R1 represents the hub, R2 and R3 are the remote spokes. Each site has a local internet connection. Our aim to provide connectivity between the LAN subnets of all three sites.

The IP addresses 1.1.1.1/30, 2.2.2.1/30 and 3.3.3.1/30 are the public IPs provided by the local ISPs at respective locations. The 192.168.X.0/24 is the LAN IP subnet at each site.