Note
In this book, we use the terms PIM join message and PIM prune message to simplify the action taken when referring to the PIM join-prune message with only a join or prune action.
The software sends join-prune messages as quickly as possible. You can filter the join-prune messages by defining a routing policy. For information about configuring the join-prune message policy please refer to Table 1-24.
PIM register: PIM register messages are unicast to the RP by designated routers that are directly connected to multicast sources. The PIM register message has the following functions:
To notify the RP that a source is actively sending to a multicast group.
To deliver multicast packets sent by the source to the RP for delivery down the shared tree.
The DR continues to send PIM register messages to the RP until it receives a register-stop message from the RP. The RP sends a register-stop message in either of the following cases:
The RP has no receivers for the multicast group being transmitted.
The RP has joined the SPT to the source but has not started receiving traffic from the source.
You can use the ip pim register-source command to configure the IP source address of register messages when the IP source address of a register message is not a uniquely routed address to which the RP can send packets. This situation might occur if the source address is filtered so that the packets sent to it are not forwarded or if the source address is not unique to the network. In these cases, the replies sent from the RP to the source address will fail to reach the DR, resulting in Protocol Independent Multicast sparse mode (PIM-SM) protocol failures.
PIM requires that multicast entries are refreshed within a 3.5-minute timeout interval. The state refresh ensures that traffic is delivered only to active listeners, and it keeps routers from using unnecessary resources.
To maintain the PIM state, the last-hop DR sends join-prune messages once per minute. State creation applies to both (*, G) and (S, G) states as follows:
(*, G) state creation example: An IGMP (*, G) report triggers the DR to send a (*, G) PIM join message toward the RP.
(S, G) state creation example: An IGMP (S, G) report triggers the DR to send an (S, G) PIM join message toward the source.
If the state is not refreshed, the PIM software tears down the distribution tree by removing the forwarding paths in the multicast outgoing interface list of the upstream routers.
A rendezvous point (RP) is a router that you select in a multicast network domain that acts as a shared root for a multicast shared tree. You can configure as many RPs as you like, and you can configure them to cover different group ranges:
Static RP: You can statically configure an RP for a multicast group range. You must configure the address of the RP on every router in the domain. You can define static RPs for the following reasons:
To configure routers with the Anycast RP address.
To manually configure an RP on a device.
BSRs: The bootstrap router ensures that all routers in the PIM domain have the same RP cache as the BSR. You can configure the BSR to help you select an RP set from BSR candidate RPs. The function of the BSR is to broadcast the RP set to all routers in the domain. You select one or more candidate BSRs to manage the RPs in the domain. Only one candidate BSR is elected as the BSR for the domain.
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