PIM-DM (Dense Mode)
Introduction
Introduction
Feature Default Menu CLI Web
Configure PIM Global n/a — 3-12 —
Configure PIM VLAN Interface n/a — 3-15 —
Display PIM Route Data Disabled — 3-23 —
Display PIM Status 0 (Forward All) — 3-28 —
In a network where IP multicast traffic is transmitted for multimedia applica-
tions, such traffic is blocked at routed interface (VLAN) boundaries unless a
multicast routing protocol is running. Protocol Independent Multicast (PIM)
is a family of routing protocols that form multicast trees to forward traffic
from multicast sources to subnets that have used a protocol such as IGMP to
request the traffic. PIM relies on the unicast routing tables created by any of
several unicast routing protocols to identify the path back to a multicast
source (reverse path forwarding, or RPF). With this information, PIM sets up
the distribution tree for the multicast traffic. The PIM-DM and PIM-SM proto-
cols on the switches covered by this manual enable and control multicast
traffic routing.
IGMP provides the multicast traffic link between a host and a multicast router
running PIM-DM or PIM-SM. IGMP and either PIM-DM or PIM-SM must be
enabled on VLANs whose member ports have directly connected hosts with
a valid need to join multicast groups.
PIM-DM is used in networks where, at any given time, multicast group mem-
bers exist in relatively large numbers and are present in most subnets. PIM-
SM (described in Chapter 4 of this guide) is used in networks where multicast
sources and group members are sparsely distributed over a wide area and can
result in unnecessary multicast traffic on routers outside the distribution
paths needed for traffic between a given multicast source and the hosts
belonging to the multicast group. In such networks, PIM-SM can be used to
reduce the effect of multicast traffic flows in network areas where they are
not needed. And because PIM-SM does not automatically flood traffic, it is a
logical choice in lower bandwidth situations such as WAN environments.
3-3
