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Wednesday, 9 March 2016

What is the use of Agent Group Multicast Address and Master Group Multicast Address in VSAN?

Each Virtual SAN node has one of three roles: Master, Backup and Agent. Among the Master’s responsibilities is disseminating Virtual SAN Metadata (the Virtual SAN Datastore). The Backup keeps an eye on the Master and is ready to take over if the Master becomes unavailable. Every other Virtual SAN node in the same Virtual SAN cluster is an Agent. You can use esxcli vsan cluster get command to know which esxi is hosting Master, Backup and Agent role?

Master
The Master communicates with every other member of the same Virtual SAN cluster. One of the communications takes place over Unicast. The Master will send Unicast traffic to all Agents and the Backup over TCP port 2233. The interesting part here is that the Master has two Unicast flows going with each other node. One flow is initiated by the Master using the destination TCP port 2233. Other nodes, also using the destination TCP port 2233, initiate the other Unicast flow towards the Master.
The Master also communicates with everyone (Agents and Backup) using the Agent Group Multicast Address with a destination UDP port of 23451. This communication is used to update Metadata with all Virtual SAN cluster members. The default Agent Group Multicast Address is 224.2.3.4 Both the Agent Group Multicast address and destination UDP port can be changed in the cli using the command
The Master listens to both the Agent Group Multicast Address and the Master Group Multicast Address.


Backup
The majority of the Backup Unicast communication takes place with the Master, although some infrequent Unicast traffic can flow between the Backup and some Agents. The Backup communicates with the Master using the dual-flow Unicasts using TCP port 2233.

The Backup also communicates with everyone (Agents and Master) using the Agent Group Multicast Address with a destination UDP port of 23451. Minor detail: the Backup is also kind of an Agent.
The Backup also communicates with the Master using the Master Group Multicast Address with a destination UDP port of 12345. The default Master Group Multicast Address is 224.1.2.3. Both the Master Group Multicast Address and destination UDP port can be changed in the cli using the command

The Backup listens to both the Agent Group Multicast Address and the Master Group Multicast Address.

Agents
The Agents communicate via Unicast with both the Master and the Backup, although the communication with the Backup is infrequent.
The Agents will send updates to the Agent Group Multicast Address with a destination port of 23451.
The Agents will listen to the Agent Group Multicast Address.


How to Check Agent Group Multicast Address and Master Group Multicast Address in VSAN?
esxcli vsan network list