VNI Ranges: What do they do ?

Deployment tools for Openstack have become very popular, including the very well known Openstack-Ansible. It makes deploying a Cloud an easy task, at the expense of losing access to the insights of “Behind the Scenes” of your your Cloud deployment. If you have had to configure neutron manually, you would have come across the following section in the ml2 configuration

[ml2_type_vxlan] # 
(ListOpt) Comma-separated list of <vni_min>:<vni_max> 
tuples enumerating # ranges of VXLAN VNI IDs that are available for 
tenant network allocation. # 
# vni_ranges =

You probably have set it to a range, similar to 10:100 or 10:300 and so on

But what does this configuration mean ?

When you configure neutron to use VXLAN as the segmentation network, each tenant network gets assigned a Virtual Network Identifier “VNI”. VNIs are numeric values that you specify their range with the vni_ranges parameter. 

An advantage of having control on this parameter is that you can specify the maximum number of VXLANs that the ml2 agent can use. Although this seems like an advantage, it can also be a disadvantage in a dynamic environment as you can run into situations where your networks can not be created because all allowed VNIs are consumed. If that’s the case, you will get an error similar to the following in neutron logs

Unable to create the network. No tenant network is available for allocation."

If you get that error, it means you need to increase the available ranges and restart the services to get it updated. 

Best of Luck !