Multicasting is an important topic on your BSCI, CCNP, CCIE and exams, and can also be very confusing if you are studying for the first time. Multicasting uses concepts that are different from what you do in studying routing protocol, and that you can throw at first. I speak from experience that multicasting is like any other technology from Cisco - learn the basics mastered the basics, and then build your knowledge on this basis.
This fundamental is the RPFControl, or Reverse Path Forwarding Check.
A fundamental difference between cellular and multicasting is that a unicast to be routed to the target, while a multicast to be positioned away from the source.
"depending on the destination" and "far from the source" seems the same thing, but they are not. A unicast follow a single path from source to destination. The only factor that the router is the destination IP address - IP sourceAddress is not a factor.
With multicast routing, the destination is a multicast group of IP addresses. And 'the multicast router, with the task of deciding which roads lead to the source (upstream) and how it is downstream from the source. Reverse Path Forwarding refers to the behavior of the router sending multicast packets from the source, rather than on a specific target.
The RPF check is designed to run against any incoming multicast packet. Checks multicast routerInterface that the packet has arrived. If the package on an upstream interface - an interface that can be found on the reverse path to the source of the gate - the packet passes the check and forwarded it. If the package is to any other interface, the packet is discarded.
The RPF is used to verify the integrity of the network multicasting and also serves to remind that the basic operation of multicasting is very different from unicasting!
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