Multicast PIM Designated Router sparse mode
PIM Designated Router is a multicast feature that is used to eliminate duplicate PIM join messages that are received by multicast routers in the same segment, from being forwarded upstream to the RP, resulting in duplicate multicast streams. PIM Designated Router is only useful in a multicast environment where sparse mode is being used.
When there are two or more multicast routers within the same multi-access network (same network segment or broadcast domain), the DR election process determines which router becomes the DR. The DR will be the router that forwards the PIM join message to the RP.
If dense mode is being used, PIM Designated Router is not useful since there is no RP, and thus has no affect on the topology. The only exception is when you use IGMPv1. In such a case, the PIM DR will operate as the IGMP query router because IGMPv1 doesn't have a query router election.
The DR should not be confused with the PIM forwarder. The DR is the router that forwards the PIM join messages to the RP whereas the PIM forwarder is the router that forwards the actual multicast traffic. The PIM forwarder is determined using the PIM Assert feature .