Cloud providers often prevent you from using L2 protocols such as ARP. These protocols however are heavily used in existing software such as keepalived. This can make it hard for to move certain workloads to the cloud. This blog post demonstrates a method for creating L2 connectivity between Virtual Machines (VMs) running in GCP. The method used relies on VXLAN to create an L2 mesh between all the VMs.

In this blog post, we'll be creating the 2 VMs, named vm-1 and vm-2. The VMs will be launched on the default VPC network. Each of the VMs will have an additional vxlan0 interface, this interface we'll be using the 10.200.0.0/24 subnet.

1. Create the VMs

In this section you will create 2 Ubuntu 20.04 VMs

  1. Let's start by creating vm-1

    gcloud compute instances create vm-1 \
              --image-family=ubuntu-2004-lts --image-project=ubuntu-os-cloud \
              --zone=us-central1-a \
              --boot-disk-size 20G \
              --boot-disk-type pd-ssd \
              --can-ip-forward \
              --network default \
              --machine-type n1-standard-2
    
  2. Repeat the same command creating vm-2 this time:

    gcloud compute instances create vm-2 \
              --image-family=ubuntu-2004-lts --image-project=ubuntu-os-cloud \
              --zone=us-central1-a \
              --boot-disk-size 20G \
              --boot-disk-type pd-ssd \
              --can-ip-forward \
              --network default \
              --machine-type n1-standard-2
    
  3. Verify that SSH to both VMs is available and up. You might need o be patient.

    gcloud compute ssh root@vm-1 --zone us-central1-a --command "echo 'SSH to vm-1 succeeded'"
    gcloud compute ssh root@vm-2 --zone us-central1-a --command "echo 'SSH to vm-2 succeeded'"
    

2. Setup VXLAN mesh between the VMs

In this section, you will be creating the VXLAN mesh between vm-1 and vm-2 that you just created.

  1. Create bash variables that will be used for setting up the VXLAN mesh

    VM1_VPC_IP=$(gcloud compute instances describe vm-1 \
                   --format='get(networkInterfaces[0].networkIP)')
    VM2_VPC_IP=$(gcloud compute instances describe vm-2 \
                   --format='get(networkInterfaces[0].networkIP)')
    echo $VM1_VPC_IP
    echo $VM2_VPC_IP
    
  2. Create the VXLAN device and mesh on vm-1

    gcloud compute ssh root@vm-1 --zone us-central1-a  << EOF
    set -x
    ip link add vxlan0 type vxlan id 42 dev ens4 dstport 0
    bridge fdb append to 00:00:00:00:00:00 dst $VM2_VPC_IP dev vxlan0
    ip addr add 10.200.0.2/24 dev vxlan0
    ip link set up dev vxlan0
    EOF
    
  3. Create the VXLAN device and mesh on vm-2

    gcloud compute ssh root@vm-2 --zone us-central1-a  << EOF
    set -x
    ip link add vxlan0 type vxlan id 42 dev ens4 dstport 0
    bridge fdb append to 00:00:00:00:00:00 dst $VM1_VPC_IP dev vxlan0
    ip addr add 10.200.0.3/24 dev vxlan0
    ip link set up dev vxlan0
    EOF
    
  4. Start a tcpdump on vm-1

    gcloud compute ssh root@vm-1 --zone us-central1-a
    tcpdump -i vxlan0 -n
    
  5. In another session ping vm-2 from vm-1 and take a look at tcpdump output. Notice the arp.

    gcloud compute ssh root@vm-1 --zone us-central1-a
    ping 10.200.0.3
    

Summary

You have setup a VXLAN mesh between 2 VMs and can now easily repeat this to more VMs. If you want to have a mesh between more VMs than for each additional VM you would need to run bridge fdp append.

This blog post wouldn't have been possible without Mikal's blog on Setting up VXLAN on Google Compute Engine.


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