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I suspect incremental deployment will be the case for many large production networks, since a forklift change is simply too risky and unrealistic.
Check the following resource that describes the Multicast Forwarding Architecture
The Network
The network topology can be viewed as a core network with two types of edges, the sending edge where IP multicast streams are injected into the network, the receiving edge is where the multicast streams are consumed. The core connects the sending and receiving edges. (This is an oversimplified view of a production network but should suffice for this use case).
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