Troubleshooting Neutron networking

Neutron is a bit of a special case among the OpenStack components because it relies on and manages a fairly intricate collection of transport resources. These may be created as a result of Neutron resources being defined by end users. There is not always a straightforward correlation at first sight. Let's walk through the traffic flow for an instance to make sure that you know which agent is doing what within the Neutron infrastructure.

The first thing that an end user will do before launching an instance is create a network specific to their tenant for their instances to attach to. At the system level, this translates into a network namespace being created on the node that is running the Neutron DHCP agent. Network namespaces are virtual network spaces that are isolated from the host-level networking. This is how Neutron is able to do isolated networks per tenant. They all get their own network namespace. You can list the network namespaces on any...