Configuring a Designated Router for PIM
Configuring Interface Priority for PIM Designated Router Selection
A designated router (DR) sends periodic join messages and prune messages toward a group-specific rendezvous point (RP) for each group for which it has active members. When a Protocol Independent Multicast (PIM) router learns about a source, it originates a Multicast Source Discovery Protocol (MSDP) source-address message if it is the DR on the upstream interface.
By default, every PIM interface has an equal probability (priority 1) of being selected as the DR, but you can change the value to increase or decrease the chances of a given DR being elected. A higher value corresponds to a higher priority, that is, greater chance of being elected. Configuring the interface DR priority helps ensure that changing an IP address does not alter your forwarding model.
DR priority is specific to PIM sparse mode; as per RFC 3973, PIM DR priority cannot be configured explicitly in PIM Dense Mode (PIM-DM) in IGMPv2 – PIM-DM only support DRs with IGMPv1.
To configure the interface designated router priority:
See Also
Configuring PIM Designated Router Election on Point-to-Point Links
In PIM sparse mode, enable designated router (DR) election on all PIM interfaces, including point-to-point (P2P) interfaces. (DR election is enabled by default on all other interfaces.) One of the two routers might join a multicast group on its P2P link interface. The DR on that link is responsible for initiating the relevant join messages. (DR priority is specific to PIM sparse mode; as per RFC 3973, PIM DR priority cannot be configured explicitly in PIM Dense Mode (PIM-DM) in IGMPv2 – PIM-DM only support DRs with IGMPv1.)
To enable DR election on point-to-point interfaces: