Designated Router

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

• The receiver's DR sends PIM join and PIM prune messages from the receiver network toward the RP.
• The source's DR sends PIM register messages from the source network to the RP.

Regardless of whether it is the receiver's DR or the source's DR, a DR is selected from other routers in a network by the exchange of IP addresses. Neighboring PIM sparse-mode 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.