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I have seen a lot of people saying that OpenDaylight is the best SDN controller for OpenStack. While I understand the benefits of a software defined network, I can't see how ODL is better than Neutron in an OpenStack cloud.

In both cases, the Open vSwitch are automatically configured (either by Neutron or ODL), and the end user just use the regular OpenStack online interface or the command line to create his networks and VMs.

So, why is the reason people add an SDN controller, especially OpenDaylight, in a cloud like OpenStack ?

Thank in advance.

Nakrule
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First, let me clarify that Neutron is only an API layer and always needs a backend (configured as a Neutron plug-in) service in order to implement the actual networking for the OpenStack cloud. In many cases, the Open vSwitch (OVS) plug-in is configured out of the box, and people are mixing Neutron and the actual OVS-based Neutron implementation.

To answer your question: OpenStack and Neutron are all about choice. If the OVS solution for Neutron is good enough for you, then great -- you don't need an "SDN" nor OpenDaylight in the mix. But some find this solution not good enough for them, typically because of missing functionality like controlling both the virtual and physical network from one place, bridging between Neutron overlay networks (typically VXLAN VNIs) and existing networks in the data center (VLAN, IP/MPLS, etc.), connecting OpenStack projects with other (non-OpenStack) infrastructure (e.g VMware, public cloud), and so on. This is where the OVS solution is being replaced with another "SDN".

Around Icehouse time-frame, the concept of Modular Layer 2 (ML2) was introduced. Many SDN solutions plug-in into Neutron via this ML2 interface and a mechanism driver.

It should be noted that ML2 is focused on L2 Neutron resources. In order to implement L3 resources (like routing, NAT) there is a need for an L3 service plugin. Similarly, there are separate driver interfaces for L4-L7 resources such as LBaaS, VPNaaS, FWaaS, BGP/VPN and so on. So depending on the SDN solution and its capabilities you may see a combination of mechanism driver, an L3 service plug-in, and L4-L7 drivers. As an example, the OpenDaylight plug-in for Neutron (aka networking-odl) includes an ML2 mechanism driver, but also a bunch of other drivers for L3-L7 services, see https://github.com/openstack/networking-odl.

Nir Yechiel
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