DMVPN (Dynamic Multipoint VPN) is a technique where we use multipoint GRE tunnels instead of GRE point-to-point tunneling. These multipoint GRE tunnels will be encrypted using IPSEC so that we have a secure scalable tunneling solution. If you are unfamiliar with tunneling or IPSEC I highly recommend to check the basic configuration for GRE first and how to configure an encrypted GRE tunnel with IPSEC. Having said that let’s look at the configuration of DMVPN. This is the topology that we will use:
Let me explain this topology to you:
· R1,R2 and R3 are able to reach each other using their FastEthernet 0/0 interfaces. I used the 192.168.123.0 /24 subnet so that they can reach each other.
· R1 will be the hub router and R2/R3 will be the spoke routers.
· R2 and R3 will establish a tunnel to R1 as shown with the green dotted line.
· When R2 and R3 want to communicate with each other they will create a spoke-to-spoke tunnel as shown with the purple dotted line.
· We will use the 172.16.123.0 /24 subnet for the tunnel interfaces.
· Each router has a loopback interface with an IP address. The routers will reach each others loopback by going through the tunnel interface.
The configuration consists of a number of steps:
