TCP/IP Tutorial and Technical Overview - IBM Redbooks

In this environment, an ASBR in the MOSPF domain is configured as an inter-AS
multicast forwarder. This router is also configured with an inter-AS multicast
routing protocol. Although the MOSPF standard does not dictate the operations
of the inter-AS protocol, it does assume the protocol forwards datagrams using
RPF principles. Specifically, MOSPF assumes that a multicast datagram whose
source is outside the domain will enter the domain at a point that is advertising
(into OSPF) the best route to the source. MOSPF uses this information to
calculate the path of the datagram through the domain.
MOSPF designates an inter-AS multicast forwarder as a wildcard multicast
receiver. As with inter-area communications, this ensures that the receiver
remains on all pruned shortest-path delivery trees. They receive all multicast
datagrams, regardless of destination. Because this device has complete
knowledge of all group membership outside the AS, datagrams can be forwarded
to group members in other autonomous systems.
6.6.4 MOSPF interoperability
Routers configured to support an MOSPF network can be intermixed with
non-multicast OSPF routers. Both types of routers interoperate when forwarding
unicast data traffic. However, forwarding IP multicast traffic is limited to the
MOSPF domain. Unlike DVMRP, MOSPF does not provide the ability to tunnel
multicast traffic through non-multicast routers.
6.7 Protocol Independent Multicast (PIM)
The complexity associated with MOSPF lead to the development and
deployment of PIM. PIM is another multicast routing protocol. Unlike MOSPF,
PIM is independent of any underlying unicast routing protocol. It interoperates
with all existing unicast routing protocols.
PIM defines two modes or operation:
► Dense mode (PIM-DM), specified in RFC 3973
► Sparse mode (PIM-SM), specified in RFC 2362
Dense mode and sparse mode refer to the density of group members within an
area. In a random sampling, a group is considered dense if the probability of
finding at least one group member within the sample is high. This holds even if
the sample size is reasonably small. A group is considered sparse if the
probability of finding group members within the sample is low.
PIM provides the ability to switch between spare mode and dense mode. It also
permits both modes to be used within the same group.
Chapter 6. IP multicast 261