Networking in OpenStack : Panoramic view

This article describes the basic introduction to Openstack Neutron ,  different types of network modes and plugin available in Openstack Networking.

Neutron is an OpenStack project to provide “networking as a service” between interface devices (e.g., vNICs) managed by other Openstack services (e.g., nova).

Neutron was introduced as a core part of OpenStack with the initiative’s Folsom release. Prior to the Folsom release, networking functionality was hard-coded in the Nova compute module of OpenStack, which required developers to modify both compute and network features of OpenStack together. With Neutron, networking is a more modular element of OpenStack that can evolve independently.

The core Neutron API includes support for Layer 2 networking and IP address management(IPAM), as well as an extension for a Layer 3 router construct that enables routing between Layer 2 networks and gateways to external networks. Neutron includes a growing list of plugins that enable interoperability with various commercial and open source network technologies, including routersswitchesvirtual switches and software-defined networking (SDN)controllers.

Note:  The OpenStack Foundation changed the name of its networking project from Quantum to Neutron due to a trademark conflict with a manufacturer of tape-based data backup systems.

Why Neutron?

  • Give cloud tenants an API to build rich networking topologies, and configure advanced network policies in the cloud.
    • Example: create multi-tier web application topology
  • Enable innovation plugins (open and closed source) that introduce advanced network capabilities
    • Example: use L2-in-L3 tunneling to avoid VLAN limits, provide end-to-end QoS guarantees, used monitoring protocols like NetFlow.
  • Let anyone build advanced network services (open and closed source) that plug into Openstack tenant networks.
    • Examples: LB-aaS, VPN-aaS, firewall-aaS, IDS-aaS, data-center-interconnect-aaS.


  • Network, representing isolated virtual Layer-2 domains; a network can also be regarded as a virtual (or logical) switch;
  • Subnet, representing IPv4 or IPv6 address blocks from which IPs to be assigned to VMs on a given network are selected.
  • Port, representing virtual (or logical) switch ports on a given network. Virtual instances attach their interfaces into ports. The logical port also defines the MAC address and the IP address(es) to be assigned to the interfaces plugged into them. When IP addresses are associated to a port, this also implies the port is associated with a subnet, as the IP address was taken from the allocation pool for a specific subnet.  These could be demonstrated in the figure below :


High-level flow

  • Tenant creates a network (e.g., “net1”)
  • Tenant associates a subnet with that network (e.g., “”)
  • Tenant boots a VM, specifying a single NIC connected to “net1” (e.g.: nova boot –image <image_name> –nic net-id=<id_of_net1> <server_name>)
  • Nova contacts Neutron and creates a port1 on net1.
  • Neutron assigns an IP to port1 is assigned IP. (The IP is chosen by Neutron)
  • Tenant destroys VM.
  • Nova contacts neutron and destroys port1. Allocated IP is returned to the pool of available IP address.


Fig : High level flow

Openstack Networking Architecture:


Fig : Network Connectivity for Physical Hosts

A standard OpenStack Networking setup has up to four distinct physical data center networks:

  • Management network. Used for internal communication between OpenStack Components.   IP addresses on this network should be reachable only within the data center.
  • Data network. Used for VM data communication within the cloud deployment.  The IP addressing requirements of this network depend on the OpenStack Networking plugin being used.
  • External network. Used to provide VMs with Internet access in some deployment scenarios.  IP addresses on this network should be reachable by anyone on the Internet.
  • API network. Exposes all OpenStack APIs, including the OpenStack Networking API, to tenants. IP addresses on this network should be reachable by anyone on the Internet. The API network may be the same as the external network, because it is possible to create an external-network subnet that is allocated IP ranges that use less than the full range of IP addresses in an IP block.

Network modes in Openstack :

  • Flat mode
  • Flat DHCP mode
  • VLAN DHCP mode

Flat mode

Flat mode is the simplest networking mode. Each instance receives a fixed IP from the pool. All instances are attached to the same bridge (br100) by default. The bridge must be configured manually. The networking configuration is injected into the instance before it is booted. And there is no floating IP feature in this mode.


Fig : FlatManager network topology

Flat DHCP mode

This is similar to the flat mode in that all instances are attached to the same bridge. In this mode Nova does a bit more configuration; it will attempt to bridge into an Ethernet device (eth0 by default). It will also run dnsmasq as a dhcpserver listening on this bridge. Instances receive their fixed IPs by doing a dhcpdiscover. Moreover, floating IP feature is provided.


Fig: FlatDHCPManager – network topology

VLAN Network Mode

It is the default mode for Nova. It provides a private network segment for each project’s instances that can be accessed via a dedicated VPN connection from the Internet.

In this mode, each project gets its own VLAN, Linux networking bridge, and subnet. The subnets are specified by the network administrator, and are assigned dynamically to a project when required. A DHCP Server is started for each VLAN to pass out IP addresses to VM instances from the subnet assigned to the project. All instances belonging to one project are bridged into the same VLAN for that project.


Fig: VLANManager – network topology

The following plugins are currently included in the OpenStack Networking distribution:

Plugins can have different properties for hardware requirements, features, performance, scale, or operator tools. Because OpenStack Networking supports a large number of plugins, the cloud administrator is able to weigh different options and decide which networking technology is right for the deployment.


About Romil Gupta
" Learn , Earn , Discover and share the knowledge ". I feel enthusiastic in working on Openstack Cloud, programming in Python.

7 Responses to Networking in OpenStack : Panoramic view

  1. allinhtml says:

    Nice one

  2. hari says:

    Good document thanks.
    Can you also plz explain nova network? what is the difference between nova network and Neutron?

  3. Mithil Arun says:

    Hi! Great work with the blog.

    Could you explain the flow of code for creation of a network when done through Horizon?

  4. Srinivasa T N says:

    An hint which informs where does the plugin fit would have helped, say, for reference

  5. Harikrishna says:

    Nice Explanation..!! can you explain a the role of the neutron and what are the various steps that undego in neutron when we try to luanch the VM

  6. Mohammed Moin says:

    Nice Content.
    Pls explain how to setup multi-node environment as shown in above figure network connectivity diagram

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