It is probably still the most widely used multicast routing (although PIM - also by Steve Deering as a CISCO employee - is available, you have to pay big money for it, while DVMRP is free).
It's very straightforward as illustrated in the follows set of examples:
The multicast message will be forwarded using the Reverse Path Broadcast table (source = A):
- A router that receives the multicast must create a prune table for each multicast group
- Initially, the table will be empty (no links pruned).
(Note that the router may still need to forward multicast messages to host on its subset !)
A leaf router can be:
- A router that do not have any routers listed in its Reverse Path Broadcast forwarding table, or
- All output links in its Reverse Path Broadcast forwarding table has been pruned
In this example, the leave routers are B and C (they can tell this fact because their RPB table has no entry for source A - so they do not need to forward multicast messages from A further down the multicast tree... they must be the leaves in the multicast tree...)
Because router B serves multicast members (it can tell this because it received IGMP-join messages), it does not send prune messages.
Router C is a leaf node and does not serve any multicast receiver of group G, so it send a prune message to E (it's upstream node):
So E sends a prune messages to its upstream node:
- There are no further prune messages because all leaf routers serve multicast receivers from group G.
- The resulting multicast tree will forward messages on a "pruned" reverse path broadcast tree.
