The control plane runs in the NSX Controller cluster. The NSX Controller is an advanced distributed state management system that provides control plane functions for logical switching and routing functions. It is the central control point for all logical switches within a network and maintains information about all hosts, logical switches (VXLANs), and distributed logical routers.
The NSX Controller cluster is responsible for managing the distributed switching and routing modules in the hypervisors. The controller does not have any dataplane traffic passing through it. Controller nodes are deployed in a cluster of three members to enable high-availability and scale. Any failure of the controller nodes does not impact any data-plane traffic.
NSX Controllers work by distributing network information to hosts. To achieve a high level of resiliency the NSX Controller is clustered for scale out and HA. NSX Controller cluster must contain three nodes. The three virtual appliances provide, maintain, and update the state of all network functioning within the NSX domain. NSX Manager is used to deploy NSX Controller nodes.
The three NSX Controller nodes form a control cluster. The controller cluster requires a quorum (also called a majority) in order to avoid a "split-brain scenario." In a split-brain scenario, data inconsistencies originate from the maintenance of two separate data sets that overlap. The inconsistencies can be caused by failure conditions and data synchronization issues. Having three controller nodes ensures data redundancy in case of failure of one NSX Controller node.
A controller cluster has several roles, including:
Each role has a master controller node. If a master controller node for a role fails, the cluster elects a new master for that role from the available NSX Controller nodes. The new master NSX Controller node for that role reallocates the lost portions of work among the remaining NSX Controller nodes.
NSX Data Center for vSphere supports three logical switch control plane modes: multicast, unicast and hybrid. Using a controller cluster to manage VXLAN-based logical switches eliminates the need for multicast support from the physical network infrastructure. You don’t have to provision multicast group IP addresses, and you also don’t need to enable PIM routing or IGMP snooping features on physical switches or routers. Thus, the unicast and hybrid modes decouple NSX from the physical network. VXLANs in unicast control-plane mode do not require the physical network to support multicast in order to handle the broadcast, unknown unicast, and multicast (BUM) traffic within a logical switch. The unicast mode replicates all the BUM traffic locally on the host and requires no physical network configuration. In the hybrid mode, some of the BUM traffic replication is offloaded to the first hop physical switch to achieve better performance. Hybrid mode requires IGMP snooping on the first-hop switch and access to an IGMP querier in each VTEP subnet.