Prior to the inclusion of the Modular Layer 2 (ML2) plugin in the Havana release of OpenStack, Neutron was limited to using a single core plugin. This design resulted in homogenous network architectures that were not extensible. Operators were forced to make long-term decisions about the network stack that could not easily be changed in the future. The ML2 plugin, on the other hand, is extensible by design and supports heterogeneous network architectures that can leverage multiple technologies simultaneously. The ML2 plugin replaced two monolithic plugins in its reference implementation: the Linux bridge core plugin and the Open vSwitch core plugin.