How to Build an OpenStack Network using the OpenStack CLI



Within this article we will detail the steps required to build a simple Neutron networking topology using the OpenStack CLI.


Our topology (Figure 1) will consist of an L3 router, an external network, a tenant network and a range of floating IPs. Our external network will be a VLAN based network and segment traffic using a VLAN tag of 50.

Figure 1 – Topology



Before we dive into the configuration steps, lets quickly look at each of the components that we will use to build our topology,

  • External network – The external network allows for ‘external’ connectivity to the neutron (i.e L3) router. In turn allowing (via floating IPs) access into the instance.
  • Tenant network – The network that your virtual instances will reside on.
  • L3 Router – An L3 router, much like a physical router provides connectivity between networks and is also able to perform network address translation (NAT).
  • Floating IPs – Floating IPs are synonymous to Static NAT i.e they provide a 1-to-1 mapping. This provides the ability for inbound connectivity from the external network into the instance.

Physical Gateway

Before we start with configuring Neutron, the physical gateway (in our case a Cisco ASA5505) is configured to provide remote connectivity. Within our configuration we simply configure a trunk and the corresponding VLAN (i.e the provider segment aka VLAN 50).

interface Ethernet0/4
  description ## OPENSTACK TRUNK ##
  switchport trunk allowed vlan 50
  switchport trunk native vlan 1
  switchport mode trunk
  speed 100
  duplex full
interface Vlan50
  security-level 100
  ip address

External Network

So the first Neutron component we will configure, will be the external network. This will be a VLAN network, and will segment traffic using a VLAN tag of 50.

openstack network create --provider-network-type vlan \
                         --provider-physical-network vlan \
                         --provider-segment 50 \
                         --external \
                         --share \
openstack subnet create --subnet-range \
                        --network external_network \
                        --no-dhcp \
                        --gateway= \
                        --allocation-pool start=,end= \

NOTE I must point out the Provider Physical Network option.

Many of you may be asking, What do I add for ‘–provider-physical-network’ when configuring the network? This is defined within physical_interface_mappings in linuxbridge_agent.ini or bridge_mappings within openvswitch_agent.ini.

Example below,


physical_interface_mappings = flat:eth12,vlan:br-vlan

Tenant Network

Next we create the tenant network and tenant subnet.

openstack network create tenant_network
openstack subnet create tenant_subnet \
                        --subnet-range \
                        --network tenant_network

L3 Router

Finally we create the L3 router, along with assigning the external network and tenant subnet to it.

openstack router create l3_router
openstack router set l3_router --external-gateway external_network
openstack router add subnet l3_router tenant_subnet

Next Steps

With the topology configured, you can now create an instance. Ensuring you assign it to the tenant network, and then associate a floating IP to it, in order to allow connectivity to/from your instance.

Rick Donato

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