HP 3500YL - Manual (Page 113)

PIM-SM (Sparse Mode)
PIM-SM Operation and Router Types
PIM-SM Operation and Router Types
Unlike PIM-DM, PIM-SM assumes that most hosts do not want to receive
multicast traffic, and uses a non-flooding multicast model to direct traffic for
a particular multicast group from the source to the VLAN(s) where there are
multicast receivers that have joined the group. As a result, this model sends
traffic only to the routers that specifically request it.
PIM-SM Operation
In a given PIM-SM domain, routers identified as Designated Routers (DRs),
Rendezvous Points (RPs), and a Bootstrap Router (BSR) participate in deliv­
ering multicast traffic to the IP multicast receivers that request it. This
approach avoids the flooding method of distributing multicast traffic
(employed by PIM-DM) and is best suited for lower bandwidth situations.
The software supports the following operation to enable multicast traffic
delivery within a PIM-SM domain:
■ From a pool of eligible DR candidates in each VLAN, one Designated
Router (DR) is elected for each VLAN interface having at least one PIM­
SM router. In a multinetted domain, this DR supports multicast traffic
from a source on any subnet in the VLAN.
■ From a pool of eligible Bootstrap Router (BSR) candidates in the domain,
one BSR is elected for the entire domain.
■ From a pool of eligible Candidate Rendezvous Points (C-RPs), one is
elected to support each multicast group or range of groups allowed in the
domain, excluding any group supported only by static-RPs. The multicast
groups allowed in the domain are determined by the aggregation of the
groups allowed by the individually configured RPs and any static-RPs.
(Note that RP-Cs and static RP’s can be configured with overlapping
support for a given set of multicast groups.)
Rendezvous-Point Tree (RPT)
When a DR in a VLAN receives traffic for a particular multicast group from a
source on that VLAN, the DR encapsulates the traffic and forwards it to the
RP elected to support that multicast group. The RP decapsulates the traffic
and forwards it on toward the multicast receiver(s) requesting that group. This
forms a Rendezvous Point Tree (RPT) extending from the DR through any
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