Potential Issues with Multicast within a VLAN Spanning Switches
Background
I ran into an interesting issue yesterday at work. There is a new video system
being installed, which takes the video output from computers, encodes it and
sends it as multicast to a controller. The controller then displays it on
a video wall. I had been told that the network has to support multicast.
As all the devices were residing in the same VLAN, I did not expect any issues.
However, the system was not able to receive the multicast. At first we expected
it could be the virtual environment and that the vSwitch did not support multicast,
because one server was deployed on the ESX cluster. The topology was this:
Multicast at Layer 2
Before describing the issue, let’s think about how multicast at layer 2 works.
The source will send to a multicast destination IP. This IP is the converted to a
destination MAC address. If the group is 227.0.0.1, this would map to the MAC
address 0100.5e00.0001. Switches forward multicast and broadcast frames to all
ports in a VLAN. This is not effective in the case of multicast as the traffic
may not have been requested by the host connected to Continue reading