A Virtual Machine (VM), a part from the CPU and Memory, needs the Connectivity. L2 Agent (OVS in this case, or an External L2 agent) is used to connect the VM to the physical port. OVS resides on the Hypervisor of each OpenStack Node.
To understand how exactly the L2 Agent Works, and how it provides the VM connectivity to the “outside world”, we first need to get a bit “deeeper” into the Linux-y nature of the OVS, and understand all the Bridge Types, what they are used for and how they interconnect. This might look a bit complicated in the beginning, specially if you come from traditional Networking background.
These are the OVS Bridge Types:
- br-int (Integration Bridge): All the VMs use the VIF (Virtual Interfaces) to connect to the Integration Bridge.
- br-eth (Ethernet Bridge): OVS Ethernet Bridge is the entity that allows us to decide if we want to use the default VLAN, or tag the VLAN before the traffic goes out.
- br-tun: Tunel Interface, where the Headers are added depending on the Tunnel type (in case there is a tunnel, such as GRE).
- br-ex (External Bridge): Used for the interconnection with the External network, or from the OpenStack point of view – to the Public Network. Have in mind that this Public Network doesn't necessarily mean that it´s an actual Public Network (with a public IP address), its just outside of the OpenStack environment.
- veth (virtual Ethernet): Used to interconnect the “bridges” (OVS-to-OVS bridge or Linux-to-OVS bridge).
On the diagram below you may get a better representation of what is actually happening when 2 VMs on different Hypervisors communicate. Notice that these are in the same subnet (10.5.5.0/24), and focus on all the elements of their interconnection.
All this sounds simple enough, but how does it go over the Physical network? We are missing 2 parts of the Puzzle:
- A way to do an Overlay Tunneling over our Physical Network (overcome the limitation of L2 connectivity by enabling the transport of the L2 packets over the L3 Network), and the “candidate” protocols are VXLAN, GRE and STT.
- Let OpenStack Neutron configure the Physical Network.