24-5
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter24 Configuring IP Multicast Routi ng Cisco Implementation of IP Multicast Routing
group-specific query. It responds with an IGMPv2 membership report to inform Router 1 that a m ember
is still present. When Router 1 receives the report, it keeps the group active on the subnet. If no response
is received, the query router stops forwarding its traffic to the subnet.
Understanding PIM
PIM is called protocol-independent: regardless of the unicast routing protocols used to populate the
unicast routing table, PIM uses this information to perform multicast forwarding instead of maintaining
a separate multicast routing table.

PIM Versions

Two versions of PIM are supported in the IOS software. With PIM Version 1 (PIMv1), Cisco introduced
support in IOS Release 11.1(6) for a new feature called Auto-RP. This proprietary feature eliminates the
need to manually configure the rendezvous point (RP) information in every router and multilayer switch
in the network. For more information, see the “Auto-RP” section on page24-8.
Beginning with IOS Release 11.3, Cisco introduced support for PIM Version 2 (PIMv2) and its
associated bootstrap router (BSR) capability. Like Auto-RP, the PIMv2 BSR mechanism eliminates the
need to manually configure RP information in every router and multilayer switch in the network. For
more information, see the “Bootstrap Router” section on page 24-8.
All systems using Cisco IOS Release 11.3(2)T or later start in PIMv2 mode by default. PIMv2 includes
these improvements over PIMv1:
•A single, active RP exists per multicast group, with multiple backup RPs. This single RP compares
to multiple active RPs for the same group in PIMv1.
•A BSR provides a fault-tolerant, automated RP discovery and distribution m echa nis m tha t en ab les
routers and multilayer switches to dynamically learn the group -to- RP map pings .
•Sparse mode and dense mode are properties of a group, a s opp osed t o an i n terf ace . We strongly
recommend sparse-dense mode, as opposed to either sparse mode or dense mode only.
•PIM join and prune messages have more flexible encoding for multiple address families.
•A more flexible hello packet format replaces the query packet to encode current and future
capability options.
•Register messages to an RP specify whether they are sent by a border router or a designa ted rou ter.
•PIM packets are no longer inside IGMP packets; they are standalone packets.

PIM Modes

PIM can operate in dense mode (DM), sparse mode (SM), or in sparse-dense mode (PIM DM-SM),
which handles both sparse groups and dense groups at the same time.

PIM DM

In dense mode, a PIM DM router or multilayer switch assumes that all other routers or multilayer
switches forward multicast packets for a group. If a PIM DM device receives a multicast packet and has
no directly connected members or PIM neighbors present, a prune message is sent back to the source .
Subsequent multicast packets are not flooded to this router or sw it ch o n th is pr uned bra nch. PI M D M
builds source-based multicast distribution trees.