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Chapter 24 Configuring IP Multicast Routing
Cisco Implementation of IP Multicast Routing
Understanding DVMRP
Distance Vector Multicast Routing Protocol (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
multicast backbone (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
Cisco IOS 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 Cisco IOS software does not implement the complete DVMRP. The Cisco IOS software
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 Neighbor Discovery
A DVMRP router learns about other DVMRP routers by periodically sending DVMRP probe messages
to the all-DVMRP-routers multicast group (224.0.0.4). A second DVMRP router receiving the message
adds the IP address of the first router that sent the probe to its internal list of DVMRP neighbors on the
received interface and then sends its own probe message. This probe message contains all the addresses
of neighboring DVMRP routers in its neighbor list, including the address of the first router. When the
first DVMRP router receives a probe with its own address listed in the neighbor list, a two-way
adjacency is formed between itself and the neighbor that sent the probe.
DVMRP Route Table
DVMRP neighbors build a route table by periodically exchanging source network routing information
in route-report messages. These messages contain entries that advertise a source network with a mask
and a hop count that is used as the routing metric. 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 reverse-path forwarding (RPF).
DVMRP Source Distribution Tree
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. DVMRP supports a reliable graft and
graft-ack mechanism that grafts previously pruned branches of a tree. The graft-ack messages are sent
by the upstream router in response to received graft messages, preventing the loss of a graft message
because of congestion.
Understanding CGMP
This software release provides CGMP-server support on your multilayer switches; no client-side
functionality is provided. The multilayer switch serves as a CGMP server for devices that do not support
IGMP snooping but have CGMP-client functionality.
