OpenStack Neutron Components and Concepts: Page 2 of 2

Networking concepts in Neutron

It is also a good idea to get familiar with some of the concepts that we will come across while dealing with Neutron, so let's take a look at some of them. The networking provides multiple levels of abstraction:

• Network: A network is an isolated L2 segment, analogous to a VLAN in the physical networking world.
• Subnet: This is a block of IP addresses and the associated configuration state. Multiple subnets can be associated with a single network (similar to secondary IP addresses on switched virtual interfaces of a switch).
• Port: A port is a connection point to attach a single device, such as the NIC of a virtual server, to a virtual network. We have seen physical ports to which we plug our laptops or servers into them; the virtual ones are quite analogous to those, with the difference that these ports belong to a virtual switch and we connect it using a virtual wire to our servers.
• Router: A router is a device that can route traffic between different subnets and networks. Any subnets on the same router can talk to each other without a routing table if the security groups allow the connection.

In order to express this better, let's take a look at the following diagram, which shows the connectivity between the Compute node and Network nodes -- the management network is used by administrators to configure the nodes and other management activities (these networks and their purposes have been described in Chapter 1, An Introduction to OpenStack):

The Tunnel Network exists between the compute nodes and the network node and serves to build a GRE Tunnel between the two. This GRE Tunnel, as we know, encapsulates different networks created on the compute nodes, so that the physical fabric doesn't see any of it. In our configurations, we will set up the VLAN ranges for this purpose.

The network node primarily performs the Layer 3 functionality, be it routing between the different networks or routing the networks to an external network using its external interface and the OVS bridge created on the Network Node. In addition, it also performs other L3-related functions such as firewalling and load balancing. It also terminates the elastic IPs at this level.

So let's take a look at how this would look in the real world:

As you can see from the diagram, the physical network has absolutely no idea of the existence of the different VLANs, which will now be assigned to different tenants or the same tenant for different purposes. We can create as many VLANs as needed, and the underlying physical network will not be affected.

The virtual router in the network node will be responsible for routing between the different VLANs, and the router may also provide access to the Internet using another network interface in the network node (we call it the external network).

To summarize, ports are the configurations on the networks where the guest VM can be connected. The network is equivalent to the virtual switch VLAN in which there may be one or more subnets. It is on a layer 2 network domain. Different networks are connected to the router on the Network Node using a GRE Tunnel, and each network is encapsulated with a single VLAN ID for identification.

To learn more about OpenStack features, check out "OpenStack: Building a Cloud Environment," published by Packt and written by Alok Shrivastwa, Kevin Jackson, Cody Bunch, Egle Sigler, and Tony Campbell.