In a VLAN populated by one or more routers running PIM-SM, one such router is elected the DR for that VLAN. When the DR receives a Join request from a multicast receiver on that VLAN, it forwards the join toward the router operating as the RP for the requested multicast group.

Where multiple PIM-SM routers exist in a VLAN, the following criteria is used to elect a DR:

  1. The router configured with the highest DR priority in the VLAN is elected.
  2. If multiple routers in the VLAN are configured with the highest DR priority, the router having the highest IP address is elected.

In a given domain, each VLAN capable of receiving multicast traffic from a unicast source should have at least one DR. (Enabling PIM-SM on a VLAN automatically enables the router as a DR for that VLAN.) Because there is an election process for DR on each VLAN, all routers on a VLAN need to be enabled for DR. Where it is important to ensure that a particular router is elected as the DR for a given VLAN, you can increase the DR priority on that VLAN configuration for that router.

If it is necessary to prevent a router from operating as a DR on a given VLAN, disable DR operation by configuring the DR priority as zero (0.)