PIM-SM Configuration
©2008 Allied Telesis Inc. All rights reserved.
36.2
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
Software Version 5.2.1
Introduction
PIM Sparse Mode
PIM Sparse Mode (PIM-SM) provides efficient communication between members of sparsely
distributed groups - the type of groups that are most common in wide-area internetworks. It is
designed on the principle that several hosts wishing to participate in a multicast conference
does not justify flooding the entire internetwork with periodic multicast traffic. PIM-SM is
designed to limit multicast traffic so that only those routers interested in receiving traffic for a
particular group receive the traffic.
Routers with directly attached or downstream members are required to join a Sparse Mode
distribution tree by transmitting explicit join messages. If a router does not become part of the
predefined distribution tree, it does not receive multicast traffic addressed to the group. In
contrast, dense mode multicast routing protocols assume downstream group membership and
continue to forward multicast traffic on downstream links until explicit prune messages are
received. The default forwarding action of a sparse mode multicast routing protocol is to block
traffic unless it is explicitly requested, while the default action of the dense mode multicast
routing protocols is to forward traffic.
PIM-SM employs the concept of a rendezvous point (RP) where receivers “meet” sources. The
initiator of each multicast group selects a primary RP and a small ordered set of alternative RPs,
known as the RP-list. For each multicast group, there is only a single active RP. Each receiver
wishing to join a multicast group contacts its directly attached router, which in turn joins the
multicast distribution tree by sending an explicit join message to the group’s primary RP. A
source uses the RP to announce its presence and to find a path to members that have joined
the group. This model requires Sparse Mode routers to maintain some state information (the
RP-list) prior to the arrival of data packets. In contrast, Dense Mode multicast routing protocols
are data driven, since they do not define any state for a multicast group until the first data
packet arrives.
Roles in PIM Sparse Mode
A multicast sender does not need to know the addresses of the members of the group in
order to send to them, and the members of the group need not know the address of the
sender. Group membership can change at any time. When PIM is enabled on the switch, and
before the switch can route multicast traffic, it must establish which of the PIM routers in the
network are performing some key roles:
designated router
(DR),
rendezvous point
(RP), and
bootstrap router
(BSR).
Designated
router
There must be one PIM designated router (DR) in the subnetwork to which the IP hosts are
connected. Any PIM-SM interfaces on the subnetwork elect the designated router with the
highest DR priority. If there is more than one router with the same priority, or no priority, they
choose the interface with the highest IP address number. The DR performs all the PIM
functionality for the subnetwork. If the current DR becomes unavailable, the remaining routers
elect a new DR on the interface by DR priority or IP address.
Rendezvous point
Each multicast group must have a rendezvous point (RP). The RP forms the root of the group’s
distribution tree. The designated router for a multicast sender sends multicast packets towards
the RP. Designated routers with group members connected to them send join messages
towards the group’s RP. The RP candidate with the lowest priority is elected from all the RP
candidates for a group. If the RP becomes unavailable, the remaining RP candidates elect a new
RP.