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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 48 Configuring IP Multicast Routing Understanding Cisco’s Implementation of IP Multicast Routing
PIM uses both source trees and RP-rooted shared trees to forward datagrams (described in the “PIM
DM” section on page 48-4 and the “PIM-SM” section on page 48-5). The RPF check is performed
differently for each:
• If a PIM router or multilayer switch has a source-tree state (that is, an (S,G) entry is present in the
multicast routing table), it performs the RPF check against the IP address of the source of the
multicast packet.
• If a PIM router or multilayer switch has a shared-tree state (and no explicit source-tree state), it
performs the RPF check on the RP address (which is known when members join the group).
Sparse-mode PIM uses the RPF lookup function to decide where it needs to send joins and prunes:
• (S,G) joins (which are source-tree states) are sent toward the source.
• (*,G) joins (which are shared-tree states) are sent toward the RP.
DVMRP and dense-mode PIM use only source trees and use RPF as previously described.
Understanding DVMRP
DVMRP is implemented in the equipment of many vendors and is based on the public-domain mrouted
program. This protocol has been deployed in the MBONE and in other intradomain multicast networks.
Cisco routers and multilayer switches run PIM and can forward multicast packets to and receive from a
DVMRP neighbor. It is also possible to propagate DVMRP routes into and through a PIM cloud. The
software propagates DVMRP routes and builds a separate database for these routes on each router and
multilayer switch, but PIM uses this routing information to make the packet-forwarding decision. The
software does not implement the complete DVMRP. However, it supports dynamic discovery of DVMRP
routers and can interoperate with them over traditional media (such as Ethernet and FDDI) or over
DVMRP-specific tunnels.
DVMRP neighbors build a route table by periodically exchanging source network routing information
in route-report messages. The routing information stored in the DVMRP routing table is separate from
the unicast routing table and is used to build a source distribution tree and to perform multicast forward
using RPF.
DVMRP is a dense-mode protocol and builds a parent-child database using a constrained multicast
model to build a forwarding tree rooted at the source of the multicast packets. Multicast packets are
initially flooded down this source tree. If redundant paths are on the source tree, packets are not
forwarded along those paths. Forwarding occurs until prune messages are received on those parent-child
links, which further constrain the broadcast of multicast packets.
Understanding CGMP
This software release provides CGMP-server support on your switch; no client-side functionality is
provided. The switch serves as a CGMP server for devices that do not support IGMP snooping but have
CGMP-client functionality.
CGMP is a protocol used on Cisco routers and multilayer switches connected to Layer 2 Catalyst
switches to perform tasks similar to those performed by IGMP. CGMP permits Layer 2 group
membership information to be communicated from the CGMP server to the switch. The switch can then
can learn on which interfaces multicast members reside instead of flooding multicast traffic to all switch
interfaces. (IGMP snooping is another method to constrain the flooding of multicast packets. For more
information, see Chapter 26, “Configuring IGMP Snooping and MVR.”)