Nomad local multicast is designed to increase the efficiency and scalability of Nomad by allowing the download to be multicast on the branch LAN. This feature is provided in addition to the standard Nomad mechanisms for sharing the download.
The package is advertised to a branch, and one machine is elected as the master. As other machines receive the advertisement, they identify the master machine and register to the multicast group for that package. The master machine then downloads and distributes the package data via multicast.
In a perfect environment, all Configuration Manager client machines will receive the advertisement at the same time. A single machine would be elected as master and it would multicast the entire download which would be received by all machines. However, real environments are much more chaotic. The Configuration Manager clients may receive the advertisements at different times and it is possible that some machines may be offline. The Nomad master machine receiving the download may be accidentally switched off during a software download by a user. Also, in busy networks, it is possible for some multicast network traffic to be ignored. Nomad multicast has been designed to compensate for all these scenarios.
Will I benefit from using local multicast?
Due to the nature of networks and their variance in terms of setup and usage the answer to this can vary, but a general rule of thumb is: if the local distribution of the download on the branch takes longer than the download to the branch you should try to use local multicast.
Determining this will depend on the speed of your WAN link, the utilization of your WAN link for the download, the speed of your local network, the utilization of your local network for transfer, the size of the packet being downloaded and the number of users being distributed to.
So, for a 256k WAN link at 80% work rate with a local network with 100 Mbits at a utilization of 30%, it works out that if you have 90 clients or over you should be using multicast - regardless of the size of package being downloaded. If you have fewer than 90 clients on a 256k WAN link then it is unlikely the speed of your software distribution can be improved by using multicast, however the load on your master machine and your LAN network utilization could be reduced by up to ninety times. Of course, as your WAN link capacity increases, the time taken for local redistribution has a greater effect on the overall timing, so the number of clients required for a recommendation of multicast decreases.