Setting up a overlay network using point to multi-point GRE tunnels

Multi-Point GRE Tunnels

Typically VxLAN is used to setup a point to multi-point overlay networks. However GRE also offers a similar capability which is often not heavily used. This document calls out how to setup an effient overlay mesh network to link up multiples sits together using multi point GRE tunnels.

Setup

The setup used in this case assumes there is a need to link three sites hosting three different subnets together using a simple overlay mesh network.

• Node A: 192.168.33.1 hosting subnet 10.0.1.0/24
• Node B: 192.168.33.2 hosting subnet 10.0.2.0/24
• Node C: 192.168.33.3 hosting subnet 10.0.3.0/24

We desire seamless connectivity between all the three subnets 10.0.1.0/24, 10.0.2.0/24 and 10.0.3.0/24.

Setting up the overlay network

Setup the multi-point GRE tunnel

On Node A:

ip tunnel add overNet mode gre local 192.168.33.1 key 1234

Assign an IP Address to the tunnel end point

ip addr add 10.0.0.1/24 dev overNet

Add neighbour entries for remote end points

ip neighbor add 10.0.0.2 lladdr 192.168.33.2 dev overNet
ip neighbor add 10.0.0.3 lladdr 192.168.33.3 dev overNet
ip link set dev overNet up

Note: The link layer address here is the IP address of the remote node and not a IP addresses

Add routes to reach the remote subnets via the tunnel end points

ip route add 10.2.0.0/24 via 10.0.0.2
ip route add 10.3.0.0/24 via 10.0.0.3

Note: We route the traffic through the overlay tunnels

Repeat these steps on Node B and Node C, with appropriate changes.

Node B Setup

ip tunnel add overNet mode gre local 192.168.33.2 key 1234
ip addr add 10.0.0.2/24 dev overNet
ip neighbor add 10.0.0.1 lladdr 192.168.33.1 dev overNet
ip neighbor add 10.0.0.3 lladdr 192.168.33.3 dev overNet
ip link set dev overNet up
ip route add 10.1.0.0/24 via 10.0.0.1
ip route add 10.3.0.0/24 via 10.0.0.3

Node C Setup

ip tunnel add overNet mode gre local 192.168.33.3 key 1234
ip addr add 10.0.0.3/24 dev overNet
ip neighbor add 10.0.0.2 lladdr 192.168.33.2 dev overNet
ip neighbor add 10.0.0.3 lladdr 192.168.33.3 dev overNet
ip link set dev overNet up
ip route add 10.2.0.0/24 via 10.0.0.2
ip route add 10.3.0.0/24 via 10.0.0.3

Now you will be able to reach all the subnets seamlessly with Node A, Node B and Node C acting as a routers and for traffic traversing across subnets