The long-promised unicast-only VXLAN has finally shipped with the Nexus 1000V release 4.2(1)SV2(2.1) (there must be some logic behind those numbers, but they all look like madness to me). The new Nexus 1000V release brings two significant VXLAN enhancements: unicast-only mode and MAC distribution mode.
The initial VXLAN design and implementation took the traditional doing-more-with-less approach: VXLANs behave exactly like VLANs (including most of the scalability challenges VLANs have) and rely on third-party tool (IP multicast) to solve the hard problems (MAC address learning) that both Nicira and Microsoft solved with control-plane solutions.
Unicast-only VXLAN comes closer to what other overlay virtual networking vendors are doing: the VSM knows which VEMs have VMs attached to a particular VXLAN segment and distributes that information to all VEMs – each VEM receives a per-VXLAN list of destination IP addresses to use for flooding purposes.
MAC Distribution Mode
MAC distribution mode goes a step further: it eliminates the process of data-plane MAC address learning and replaces it with control-plane solution (similar to Nicira/VMware NVP) – VSM is collecting the list of MAC addresses and distributing the MAC-to-VTEP mappings to all VEMs participating in a VXLAN segment.
Cisco also increased the maximum number of VEMs a single VSM can control to 128, and the maximum number of virtual ports per VSM (DVS) to 4096.
Does It Matter?
Sure it does. The requirement to use IP multicast to implement VXLAN flooding was a major showstopper in data centers that have no other need for IP multicast (almost everyone apart from financial institutions dealing with multicast-based market feeds). Unicast-only VXLAN will definitely simplify VXLAN deployments and increase its adoption.
MAC distribution mode is a nice-to-have feature that you’d need primarily in large-scale cloud deployments. Most reasonably sized enterprise data centers can probably live happily without it (of course I might be missing something fundamental – do write a comment).
The original VXLAN proposal was a data-plane-only solution – boxes from different vendors (not that there would be that many of them) could freely interoperate as long as you configured the same IP multicast group everywhere.
Unicast-only VXLAN needs a signaling protocol between VSM (or other control/orchestration entity) and individual VTEPs. The current protocol used between VSM and VEMs is probably proprietary; Cisco claims to plan to use VXLAN over EVPN for inter-VSM connectivity, but who knows when the Nexus 1000V code will ship. In the meantime, you cannot connect a VXLAN segment using unicast-only VXLAN to a third-party gateway (example: Arista 7150).
Due to the lack of inter-VSM protocol, you cannot scale a single VXLAN domain beyond 128 vSphere hosts, probably limiting the size of your vCloud Director deployment. In multicast VXLAN environments the vShield Manager automatically extends VXLAN segments across multiple distributed switches (or so my VMware friends are telling me); it cannot do the same trick in unicast-only VXLAN environments.
Blog posts discussing VXLAN scalability and IP multicast: