Understanding Designated Routers

In a PIM sparse mode (PIM-SM) domain, there are two types of designated routers (DRs) to consider:

• The receiver DR sends PIM join and PIM prune messages from the receiver network toward the RP.

• The source DR sends PIM register messages from the source network to the RP.

Neighboring PIM routers multicast periodic PIM hello messages to each other every 30 seconds (the default). The PIM hello message usually includes a holdtime value for the neighbor to use, but this is not a requirement. If the PIM hello message does not include a holdtime value, a default timeout value (in Junos OS, 105 seconds) is used. On receipt of a PIM hello message, a router stores the IP address and priority for that neighbor. If the DR priorities match, the router with the highest IP address is selected as the DR.

If a DR fails, a new one is selected using the same process of comparing IP addresses.