4.7 Designated Routers and Hello Messages

4.7 Designated Routers and Hello Messages PIM Hello messages are sent periodically on each interface that has PIM enabled. The primary purpose of Hello messages is to announce each router's existence on the subnet as a PIM router, so all routers can decide on a single DR for the subnet. PIM Hello messages are sent on both multiaccess and point-to-point interfaces. Hello messages are sent to the multicast address 224.0.0.13 (ALL-PIM-ROUTERS group) with a TTL of 1. When a router first boots or is first configured for PIM, it sends out the initial Hello message and then sets its Hello timer to 30 seconds (the default). Each time the Hello timer counts down to 0, Hello messages are sent out, and the timer is reset. If a router does not hear from a neighbor for a period of 3.5 times the Hello timer (105 seconds is the default), the neighbor is dropped (possibly causing the election of a new DR). Note This hold-time value is actually carried in the Hello message, so routers on the same subnet can have different hold timers and not experience problems with incorrectly dropping neighbors. The Hello messages contain the configured DR priority of the router sending the message. The router with the highest DR priority is elected DR for the subnet. If any of the routers does not support the DR priority option, the DR is the router with the highest IP address. Note Each router on the subnet elects the DR, and the election results are never communicated to the other routers on the subnet. This is not a problem as long as each router has the same information and uses the same algorithm to determine the DR. If the IP address of an interface is changed, the router first sends a Hello message from the old address with a hold-time of 0, which forces the other routers on the subnet to immediately purge the old address from their neighbor tables. Then a Hello message with the new address and standard hold-time is sent. Each time the neighbor table is changed, each router runs through the DR election algorithm again. If a router loses its DR status, it no longer sends Register messages for new sources to the RP. It also stops sending (null) Registers for current sources on the subnet.