Using Multicast in Your Network
19-14 Configuring Multicast
PIM Terms and Definitions
Table 19-2 lists terms and definitions used in PIM configuration.
Join/Prune (J/P) These messages contain information on group membership
received from downstream routers.
PIM-SM adopts RPF technology in the join/prune process. When
a multicast packet arrives, the router first judges the correctness of
the arriving interfaces:
• If the packet is a source address/multicast group (S,G) entry
(on the shortest path tree (SPT)), then the correct interface is
the reverse path forwarding (RPF) interface towards the
source.
• If the packet is not an S,G entry (on the RP tree (RPT)), then
the correct interface is the RPF interface towards the RP.
A router directly connected to the hosts is often referred to as a
leaf router or DR. The leaf router is responsible for sending the
prune messages to the RP, informing it to stop sending multicast
packets associated with a specific multicast group. When the RP
receives the prune message, it will no longer forward the multicast
traffic out the interface on which it received the prune message.
Assert These messages indicate that the device received a data packet
on its outbound (receiving) interface for the group. They report the
metric or distance to the source or RP to help the device identify
the most direct path to the root of the tree. If multiple routers claim
to have the most direct path to the source or RP, each device
sends its own assert message and the router with the best metric
wins. The other device will then remove that link from its outbound
interface list for the group.
Bootstrap These messages are sent by the PIM-SM router that has been
elected as the bootstrap router (BSR) to inform all PIM-SM routes
of the RP/group mappings.
Candidate RP message These messages are sent by the configured candidate RP routers
to the BSR to inform the BSR of its RP/group candidacy.
Table 19-1 PIM-SM Message Types (continued)
Message Type Description
Table 19-2 PIM Terms and Definitions
Term Definition
Bootstrap Router (BSR) A PIM router responsible for collecting, within a PIM domain, the set of potential
rendezvous points (RPs) and distributing the RP set information to all PIM
routers within the domain. The BSR is dynamically elected from the set of
candidate BSRs.
RP set information includes group-to-RP mappings.
Candidate Bootstrap
Router (Candidate-BSR)
A small number of routers within a PIM domain are configured as candidate
BSRs, and each C-BSR is given a BSR priority. All C-BSRs multicast bootstrap
messages (BSMs) containing their priority to the ALL-PIM-ROUTERS group.
When a C-BSR receives a bootstrap message from a C-BSR with a higher
priority, it stops sending. This continues until only one C-BSR remains sending
bootstrap messages, and it becomes the elected BSR for the domain.
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