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:
1. Basic configuration of IP
addresses.
2. GRE Multipoint Tunnel
configuration on all routers
3. Encryption of tunnels using
IPSEC.
4. Routing configuration so the
routers can reach each others loopback interfaces.
Let’s get
started!
