Yesterday I received the question if all VSAN Witness VMs for ROBO can be in the same VLAN? The excellent VSAN 2 node and Stretched Cluster guide for VSAN describes an example of how things can be implemented with different VLANs for the VSAN Witness, which would look like this:
Now the question came in from a customer implementing ROBO at scale if it was possible to have all VSAN Witness VMs for ROBO in the same VLAN. The answer in short is: yes. Keep in mind that the ROBO locations access the Witness VM over L3 and there is no multicast needed between the ROBO location. Only thing you need to do is set up the routes from the ROBO location to the main site with the central Witness VMs. All of them can be in the same VLAN, fully supported!