Protocol-Independent Multicast: Difference between revisions

Revision as of 19:13, 20 January 2022

Revision as of 19:50, 31 October 2022

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The basic assumption behind dense mode is that the multicast packet stream has receivers at most locations. Sparse mode assumes relatively fewer receivers. Dense mode is ideal for groups where many of the nodes will subscribe to receive the multicast packets, so that most of the [[router (computing)|router]]s must receive and forward these packets (groups of a high density).

This difference shows up in the initial behavior and mechanisms of the two protocols. Dense Mode uses a fairly simple approach to handle [[IP multicast]] routing. The source initially [[broadcastingBroadcasting (networksnetworking)|broadcast]]s to every [[router (computing)|router]] directly connected to it. These neighboring routers further forward the data to their neighbors. When a router does not wish to receive this group's data (if no other neighboring PIM routers are present and no host is interested in the group), it sends a Prune message to indicate its lack of interest. Upon receiving a Prune message, the router will modify its state so that it will not forward those packets out that [[Interface (computer science)|interface]]. If every interface on a router is pruned, the router will also be pruned.<ref name=CiscoPIM/>

In older Cisco IOS releases, PIM-DM would re-flood all the multicast traffic every 3 minutes. This is fine for low volume multicast, but not higher bandwidth multicast packet streams. More recent Cisco IOS versions support a new feature called PIM Dense Mode State Refresh, since 12.1(5)T. This feature uses a PIM state refresh messages to refresh the Prune state on outgoing interfaces. Another benefit is that topology changes are recognized more quickly. By default, the PIM state refresh messages are sent every 60 seconds.