vSAN 6.6 Cluster with Unicast Traffic

vSAN 6.6 Cluster with Unicast Traffic

There are good chances that you’d be using vSAN 6.6 today. If you have not yet upgraded your environment to vSAN 6.6 yet, there are many reasons to plan it soon.

One amongst those reasons would be, with vSAN 6.6, the requirement of multicast network traffic gets lifted off. Well, it does simplify the setup especially when we talk about configuring switches involved in vSAN traffic for multicast traffic which increases complexity for sure.

This article will explain the cluster discovery and inter host communication on unicast traffic but before we figure out the fundamentals, let’s revisit what is vSAN CMMDS ?

CMMDS (Cluster Monitoring, Membership, and Directory Services) is a vSAN cluster module which is responsible for monitoring links to the cluster.

As you know, a master node (host) and a backup node (host) are elected in a VSAN cluster. The master node is responsible for dispatching an update provided by one node to all other nodes in the cluster. In a healthy cluster where all nodes are communicating over the VSAN network. IP Multicast is a fundamental requirement of vSAN prior to v6.6. Earlier vSAN versions depended on IP multicast communication for the process of joining and leaving cluster groups as well as other intra-cluster communication services. CMMDS used multicast as a discovery protocol to find all other nodes trying to join a vSAN cluster with the same sub-cluster UUID.

SAN 6.6 now communicates using unicast for CMMDS updates. The vSAN management layer, vSAN health, now maintains a list of vSAN cluster members and pushes the list of nodes to the CMMDS layer. For hosts managed by vCenter Server, the vSAN cluster IP address list is maintained centrally and is pushed down to vSAN nodes. i.e. each vSAN node host configuration file (esx.conf) will contain a vSAN cluster UUID, vSAN node UUID, IP address and unicast port.

vSAN Cluster Details – ESXCLI
vSAN Cluster Details

When a vSAN node participating in a vSAN 6.6 enabled cluster is rebooted, the esx.conf file is read. This contains the list of participating vSAN nodes. A Unicast transmission/stream sends IP packets to a single recipient on a network.

CMMDS module on ESXi hosts maintains cluster node relationships which can be observed with following commands on respective hosts:

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