Ramon Perez, Manuel Rodriguez

Networking Considerations for Distributed Deployment of Libvirt/KVM-Based OCP Clusters


In previous blog posts, we have learned about the OCP on Libvirt project and the benefits it brings to us, with regards to the flexible deployment of OCP clusters where the nodes are virtual machines deployed on KVM.

If you remember, we started by commenting how to set up a virtual development environment for OCP agent, and then we continued by describing how to use DCI to easily install an OCP cluster based on Libvirt in a single baremetal server.

However, in both cases, we have only covered the case of one single physical server. Can we use multiple physical servers to set up a lab with multiple, distributed deployment on Libvirt-based OCP clusters? The answer is yes! And we will explain the main requirements and challenges in terms of networking in this blog post.

What would you learn after reading this blog post?

  • A solution to deploy multiple OCP clusters based on Libvirt nodes, using Distributed-CI (DCI).
  • The potential use case of this kind of solutions, currently ongoing in the Telco Partner CI team.
  • The challenges to have in mind when working with this.

Why Libvirt-based OCP clusters?

The rationale of this idea is not a chance. It's just a natural, good practice that comes up when having multiple resources that can be splitted in order to maximize its usage.

So, imagine you have several powerful physical servers to build a single cluster to deploy OpenShift. You may think that having such a powerful environment to test OCP deployments and/or workloads would be great, but in the end, sometimes (mostly when dealing with testing and troubleshooting, not being in production), it's like using a sledgehammer to crack a nut! Then, you need to ask yourself: do I really need this powerful cluster to just test OCP? Am I really using properly and efficiently the resources I am taking?

If we split the cluster and dedicate each physical server to deploy a virtualized OCP cluster, using Libvirt-based virtual machines, we would eventually benefit from the advantages of Cloud Computing, maximizing the usage of the underlying physical resources and also allowing the parallelization of your development work, as you can dedicate different OCP clusters for each to-do task.

We can summarize this process with the following picture, where the Latin expression "divide et vinces" appears in its full splendor.

From single to multiple clusters

Of course, this is not an easy work, but it is not impossible. The case that we will analyze in this blog post, which is already in place in Telco Partner CI team’s testing environment, might inspire you to adopt this kind of environments.

In our particular case, recently we received 10 brand new, nice, and powerful servers, and we decided to integrate them as part of our CI jobs. Previously, we only had one dedicated server for this purpose, so we had a lack of resources sometimes when dealing with multiple changes being tested at the same time, as this server was a bottleneck.

However, the transition towards a multi-cluster environment, using Libvirt as support to deploy the OCP clusters based on virtual nodes on each server, it really allowed us to have parallel deployments, or dedicated server for troubleshooting and testing, among others.

To summarize, our work is based on the following:

  • Distributed IPI/SNO deployment of OCP clusters fully virtualized with Libvirt.
  • 10 physical servers used for this setup.
  • Distributed-CI (DCI) is used for the automation of the deployment (using Ansible, among others).

And how can DCI help us here?

You have to consider that deploying an OpenShift cluster with a particular version, custom configuration, workloads on top, etc., is not as easy as it may seem at the beginning. And also, different users and partners may decide on using their own tools and processes to address this, as commented before, so the probability of having failures or issues and not finding proper documentation or troubleshooting guidelines to try to overcome them would increase in these cases.

Here's where DCI appears to try to make your life easier. Just highlighting some concepts from its own workflow, we have to clarify that the hardware to be used allows to use:

  • Multiple options for OCP deployment (IPI, SNO, Assisted, etc.).
  • Multiple hardware configurations (baremetal, Libvirt VMs, etc.).

So, in the end, it's a platform prepared to do this kind of integrations on a CI basis, following a pipeline-and-component-based logic to install OCP and deploy workloads on top of it, regardless of the hardware used to create the cluster.


Mainly, you would need to meet the following requirements, which are mainly estimations based on the work we have done in our own labs:

  • Enough compute resources to deploy as many VMs you want to deploy.
    • 1 VM as provisionhost. At least 16 GB RAM @ 8 vCPU.
    • 1 VM for each worker/master node to be used (SNO would only need 1 VM).
      • SNO: at least 8 GB RAM @ 6 vCPU, but more recommended to install operators and workloads.
      • Multi-node cluster: at least 16 GB RAM @ 8 vCPU per node (x2 in workers).
  • At least RHEL 8 OS.
      • RHEL subscription for the Host servers.
  • Install DCI agent packages.
  • Enable nested KVM.
  • 1 Network Interface with access to the Internet.
    • Need to plan the IP addressing in advance.

Network setup

Starting with a single server

If we check the scenarios deployed in the blog posts referenced above, we will see that we only have one server, which holds the OCP clusters and acts as a jumphost to access to the nodes.

Single server scenario

In that case, VMs are connected with an internal network for provisioning, and a NATted network is used as baremetal network, allowing the virtual machines to have connectivity towards Internet (in the case of a connected cluster like this). This is managed by a virtual router created by libvirt, which has its own dnsmasq process to provide DHCP and DNS configuration to the VMs connected to that network.

Also, you can see in the figure above that we can combine the presence of different OCP clusters in the same physical server; such as IPI and SNO, as long as we define different virtual networks to isolate them.

Moving to multiple servers

Then, what do we need to do to move to a distributed deployment by using multiple servers? Some of the challenges are:

  • Define the network layout in a flat network.
  • Allow connectivity between all clusters.
  • Set up shared network services (DHCP, DNS).
  • Have a single point to manage all cluster deployments.

The solution proposed is summarized in the next picture.

Multiple servers scenario

Essentially, we need to bear in mind the following distribution of servers:

  • One server will act as jumphost to access all VMs, including all the network configuration required (dnsmasq, DHCP, etc.).
    • The dnsmasq configuration will no longer run in the server that deploys the virtual machines; it will be delegated in the physical server acting as jumphost.
  • Other servers deploy the Libvirt VMs to create the OCP clusters.
    • But here, it’s not enough to use NATted networks in the baremetal network, because they can only manage outbound traffic, but not inbound.
    • In this case, a bridged network is required instead, forcing the VMs to ask for IP configuration to the jumphost (e.g. based on MAC address).

Consequently, the key points of this solution are:

  • The usage of a Linux bridge on each server, mastering the physical network interface of the server that connects it with the jumphost, which will be the slave of this Linux bridge.
    • This Linux bridge allows the layer-2 connectivity between all the virtual machines deployed in the server, but for layer-3 configuration, we need to go one step further.
  • The centralized network configuration applied from the jumphost. All the VMs will receive configuration from the jumphost.
    • Our approach is based on static IP-MAC assignment; so, if we explicitly define the MAC address to be used by each virtual machine in the virtual network that connects it to the Linux bridge, we can configure dnsmasq in the jumphost to provide a specific IP address to the virtual machine in that interface, then allowing the layer-3 connectivity.
    • Of course, this jumphost would also apply the DNS resolution for all the VMs, not only being the point of the network from which we can SSH to the virtual machines, but also allowing the virtual machines to reach resources (e.g. registries) from outside.

Deployment and automation

You know that, with DCI, it is feasible to automate this kind of deployments. For example, with dci-openshift-agent, you can do the following with each server:

  • Define all VM settings in variables to easily adjust configurations.
  • Use different files for the variables related to each OCP cluster (on each different physical server).
  • Use scripts to create and destroy VMs for clean setups.
  • Define stages for OCP pre-configuration, installation, and post-setup.

However, this is true when acting over each phyisical server isolatedly. But what about achieving a real automation of the whole lab? Our recommended option is to apply the automation in the jumphost, so that you can rely on utilities packaged on DCI that we have already covered in several blog posts, such as prefixes, dci-queue or dci-pipeline.

How to deploy at a high level

Let's briefly review, from a high level perspective, how you can deploy this kind of environments, to make you understand the final picture you will have.

Of course, the first step from your side would be to install and configure the DCI-related tools in your system, among other requirements we already commented.

Then, adjust the variables to be used by dci-openshift-agent for your deployment. We recommend you to have different inventory files for each physical server used to deploy OCP, and also a different file for each deployment type (IPI, SNO...) you may want to use.

Let's check an example for IPI deployment. For this, you can take the default inventory file from OCP on Libvirt as base, but you need to take into account the following changes, as this example is related to a single-cluster environment with internal IP addresses:

  • It would not be needed to declare apps_ip_address, api_ip_address and dns_vip_address, since they would be defined in the jumphost's dnsmasq instance. Here's a possible dnsmasq configuration for these values and other general setup:

    dhcp-range=server,,,24h   # addresses to be used by VMs
    dhcp-option=server,option:dns-server,  # the jumphost
    dhcp-option=server,option:ntp-server,  # the physical server
  • For the baremetal network, it is enough to follow the following configuration, specifying that we would use a bridged network with br0 as the master, Linux bridge.

      - name: baremetal
        bridge: br0
        forward_mode: bridge
  • Also, in the hosts, you would need to update the networks attached to each host, specifying the MAC address to be used in the virtual interface for the baremetal network. For example:

      - name: provisionhost
          - name: provisioning
            type: network
          - name: baremetal
            type: network
            mac: "52:54:00:00:02:00"
  • You need to configure the MAC addresses provided to each VM in the baremetal network in the jumphost's dnsmasq to provide the proper IP address to the VMs. For example:


With this in mind, you would be ready, from the jumphost, to:

  • Deploy the VMs in the target server, which will be ready to be used for OCP installation.
  • Install OCP, then the jumphost will have a kubeconfig available to interact with the cluster afterwards. And since the jumphost can reach all the VMs deployed in the other servers, there will not be any issue in this interaction.

Challenges faced and lessons learned

This solution may sound really great, but as we were commenting, it was not a trivial or simple integration. Mainly, when moving from single server to multiple servers, we had to deal with the following issues, already commented:

  • Usage of bridged network for machineNetwork instead of Libvirt isolated network.
  • Centralize DHCP/DNS services to assign IP addresses to the VMs and DNS resolution.

Also, just note that, depending on the OCP deployment mode, we need to check extra configurations to allow the scenario to be working fine. For example, Libvirt OCP clusters running ACM and deploy Baremetal SNO also need to use bridged networks for machineNetwork and provisioning network.

And finally, just remark that, in labs with multiple servers, you need to plan in advance your IP addressing assignment to really know the number of clusters that you will be able to use. You can have a lot of computing resources to create multiple virtual machines, but depending on the subnet size that you have available in your scenario, you may have limitations in the number of clusters that you can deploy.

Wrap up

Just to sum up, let's briefly review the proposed solution from different perspectives:

  • In particular, from partners' perspective, this work allows to speed-up testing specific OpenShift versions and workloads deployed on top of it with really flexible deployments that can be destroyed and re-created on a simple basis, while the number of clusters available can be scaled if required. This can be a useful resource for partners and customers in order to have as many OpenShift virtualized clusters as they need, with different configurations and sizing, to cater to their needs.
  • But also, from Red Hat Hybrid Cloud's perspective, this work fits in extending Red Hat to Cloud Services and Edge, as it makes the deployment of multiple OpenShift clusters more flexible and robust, as well as allowing the evolution of customer success, as this deployment can be easily reproduced in partners and customers.

In general terms, the benefits from following this integration would be the following:

  • Speed-up the process of deploying and testing the cluster.
  • Enable the possibility of having a fully automated and virtualized laboratory, with a farm of OCP clusters with virtual nodes, each of them having different configurations and being used for different purposes.
  • Maximize the resources available.

However, this solution is not without challenges to address, already reviewed in the blog post:

  • Consider that each and every team or even individuals have their own way to deploy OCP clusters on top of KVM.
  • A wrong design or implementation may lead to various implications in terms of networking, such as ensuring the isolation between clusters.

We hope this blog post is useful for you, in case of thinking about moving to this kind of solutions!

Note that this blog post is based on the following Red Hat Networking Summit presentation, which was also recorded (only for Red Hatters), in case you want to take a look.

If you need further support, don't hesitate to reach Telco CI team to discuss about your solution and to exchange ideas about the setup. We will be glad to hear from you and try to help you!