OpenStack Operations Guide (2014)

Part I. Architecture

Chapter 7. Network Design

OpenStack provides a rich networking environment, and this chapter details the requirements and options to deliberate when designing your cloud.


If this is the first time you are deploying a cloud infrastructure in your organization, after reading this section, your first conversations should be with your networking team. Network usage in a running cloud is vastly different from traditional network deployments and has the potential to be disruptive at both a connectivity and a policy level.

For example, you must plan the number of IP addresses that you need for both your guest instances as well as management infrastructure. Additionally, you must research and discuss cloud network connectivity through proxy servers and firewalls.

In this chapter, we’ll give some examples of network implementations to consider and provide information about some of the network layouts that OpenStack uses. Finally, we have some brief notes on the networking services that are essential for stable operation.

Management Network

management network (a separate network for use by your cloud operators) typically consists of a separate switch and separate NICs (network interface cards), and is a recommended option. This segregation prevents system administration and the monitoring of system access from being disrupted by traffic generated by guests.

Consider creating other private networks for communication between internal components of OpenStack, such as the message queue and OpenStack Compute. Using a virtual local area network (VLAN) works well for these scenarios because it provides a method for creating multiple virtual networks on a physical network.

Public Addressing Options

There are two main types of IP addresses for guest virtual machines: fixed IPs and floating IPs. Fixed IPs are assigned to instances on boot, whereas floating IP addresses can change their association between instances by action of the user. Both types of IP addresses can be either public or private, depending on your use case.

Fixed IP addresses are required, whereas it is possible to run OpenStack without floating IPs. One of the most common use cases for floating IPs is to provide public IP addresses to a private cloud, where there are a limited number of IP addresses available. Another is for a public cloud user to have a “static” IP address that can be reassigned when an instance is upgraded or moved.

Fixed IP addresses can be private for private clouds, or public for public clouds. When an instance terminates, its fixed IP is lost. It is worth noting that newer users of cloud computing may find their ephemeral nature frustrating.

IP Address Planning

An OpenStack installation can potentially have many subnets (ranges of IP addresses) and different types of services in each. An IP address plan can assist with a shared understanding of network partition purposes and scalability. Control services can have public and private IP addresses, and as noted above, there are a couple of options for an instance’s public addresses.

An IP address plan might be broken down into the following sections:

Subnet router

Packets leaving the subnet go via this address, which could be a dedicated router or a nova-network service.

Control services public interfaces

Public access to swift-proxy, nova-api, glance-api, and horizon come to these addresses, which could be on one side of a load balancer or pointing at individual machines.

Object Storage cluster internal communications

Traffic among object/account/container servers and between these and the proxy server’s internal interface uses this private network.

Compute and storage communications

If ephemeral or block storage is external to the compute node, this network is used.

Out-of-band remote management

If a dedicated remote access controller chip is included in servers, often these are on a separate network.

In-band remote management

Often, an extra (such as 1 GB) interface on compute or storage nodes is used for system administrators or monitoring tools to access the host instead of going through the public interface.

Spare space for future growth

Adding more public-facing control services or guest instance IPs should always be part of your plan.

For example, take a deployment that has both OpenStack Compute and Object Storage, with private ranges and available. One way to segregate the space might be as follows:   -   - subnet routers   -  - spare for networks  - - Compute node remote access controllers

                                (inc spare) - - Compute node management interfaces (inc spare) - - Swift proxy remote access controllers

                                (inc spare) - - Swift proxy management interfaces (inc spare) - - Swift storage servers remote access controllers

                                (inc spare) - - spare  -    - subnet routers  -   - Swift proxy server internal interfaces

                                (inc spare) -   - Swift object server internal interfaces

                                (inc spare)

A similar approach can be taken with public IP addresses, taking note that large, flat ranges are preferred for use with guest instance IPs. Take into account that for some OpenStack networking options, a public IP address in the range of a guest instance public IP address is assigned to thenova-compute host.

Network Topology

OpenStack Compute with nova-network provides predefined network deployment models, each with its own strengths and weaknesses. The selection of a network manager changes your network topology, so the choice should be made carefully. You also have a choice between the tried-and-true legacy nova-network settings or the neutron project for OpenStack Networking. Both offer networking for launched instances with different implementations and requirements.

For OpenStack Networking with the neutron project, typical configurations are documented with the idea that any setup you can configure with real hardware you can re-create with a software-defined equivalent. Each tenant can contain typical network elements such as routers, and services such as DHCP.

Table 7-1 discusses the networking deployment options for both legacy nova-network options and an equivalent neutron configuration.

Network deployment model



Neutron equivalent


Extremely simple topology.

No DHCP overhead.

Requires file injection into the instance to configure network interfaces.

Configure a single bridge as the integration bridge (br-int) and connect it to a physical network interface with the Modular Layer 2 (ML2) plug-in, which uses Open vSwitch by default.


Relatively simple to deploy.

Standard networking.

Works with all guest operating systems.

Requires its own DHCP broadcast domain.

Configure DHCP agents and routing agents. Network Address Translation (NAT) performed outside of compute nodes, typically on one or more network nodes.


Each tenant is isolated to its own VLANs.

More complex to set up.

Requires its own DHCP broadcast domain.

Requires many VLANs to be trunked onto a single port.

Standard VLAN number limitation.

Switches must support 802.1q VLAN tagging.

Isolated tenant networks implement some form of isolation of layer 2 traffic between distinct networks. VLAN tagging is key concept, where traffic is “tagged” with an ordinal identifier for the VLAN. Isolated network implementations may or may not include additional services like DHCP, NAT, and routing.

FlatDHCP Multi-host with high availability (HA)

Networking failure is isolated to the VMs running on the affected hypervisor.

DHCP traffic can be isolated within an individual host.

Network traffic is distributed to the compute nodes.

More complex to set up.

Compute nodes typically need IP addresses accessible by external networks.

Options must be carefully configured for live migration to work with networking services.

Configure neutron with multiple DHCP and layer-3 agents. Network nodes are not able to failover to each other, so the controller runs networking services, such as DHCP. Compute nodes run the ML2 plug-in with support for agents such as Open vSwitch or Linux Bridge.

Table 7-1. Networking deployment options

Both nova-network and neutron services provide similar capabilities, such as VLAN between VMs. You also can provide multiple NICs on VMs with either service. Further discussion follows.

VLAN Configuration Within OpenStack VMs

VLAN configuration can be as simple or as complicated as desired. The use of VLANs has the benefit of allowing each project its own subnet and broadcast segregation from other projects. To allow OpenStack to efficiently use VLANs, you must allocate a VLAN range (one for each project) and turn each compute node switch port into a trunk port.

For example, if you estimate that your cloud must support a maximum of 100 projects, pick a free VLAN range that your network infrastructure is currently not using (such as VLAN 200–299). You must configure OpenStack with this range and also configure your switch ports to allow VLAN traffic from that range.

Multi-NIC Provisioning

OpenStack Compute has the ability to assign multiple NICs to instances on a per-project basis. This is generally an advanced feature and not an everyday request. This can easily be done on a per-request basis, though. However, be aware that a second NIC uses up an entire subnet or VLAN. This decrements your total number of supported projects by one.

Multi-Host and Single-Host Networking

The nova-network service has the ability to operate in a multi-host or single-host mode. Multi-host is when each compute node runs a copy of nova-network and the instances on that compute node use the compute node as a gateway to the Internet. The compute nodes also host the floating IPs and security groups for instances on that node. Single-host is when a central server—for example, the cloud controller—runs the nova-network service. All compute nodes forward traffic from the instances to the cloud controller. The cloud controller then forwards traffic to the Internet. The cloud controller hosts the floating IPs and security groups for all instances on all compute nodes in the cloud.

There are benefits to both modes. Single-node has the downside of a single point of failure. If the cloud controller is not available, instances cannot communicate on the network. This is not true with multi-host, but multi-host requires that each compute node has a public IP address to communicate on the Internet. If you are not able to obtain a significant block of public IP addresses, multi-host might not be an option.

Services for Networking

OpenStack, like any network application, has a number of standard considerations to apply, such as NTP and DNS.


Time synchronization is a critical element to ensure continued operation of OpenStack components. Correct time is necessary to avoid errors in instance scheduling, replication of objects in the object store, and even matching log timestamps for debugging.

All servers running OpenStack components should be able to access an appropriate NTP server. You may decide to set up one locally or use the public pools available from the Network Time Protocol project.


OpenStack does not currently provide DNS services, aside from the dnsmasq daemon, which resides on nova-network hosts. You could consider providing a dynamic DNS service to allow instances to update a DNS entry with new IP addresses. You can also consider making a generic forward and reverse DNS mapping for instances’ IP addresses, such as


Armed with your IP address layout and numbers and knowledge about the topologies and services you can use, it’s now time to prepare the network for your installation. Be sure to also check out the OpenStack Security Guide for tips on securing your network. We wish you a good relationship with your networking team!