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24-port NWay Ethernet Switch User’s Guide
Switch Management and Operating Concepts 113
the router from which it received the multicast packet. The
prune message indicates that the multicast packets of the that
particular source group member pair should not be forwarded on
the link from which the prune message has been received. It is
important to note that prune messages are only sent one hop
back towards the source. The upstream routers is required to
record the prune information in its memory. On the other hand,
if the upstream router does not have any local recipient and
receives prune messages form all of its children in the TRPB tree,
the upstream router will send a prune message itself to its
parent in the TRPB tree indicating that the multicast packets
will be forwarded only on those links that will lead to a
destination node (a link with a multicast group member). An
example of a tree obtained after the exchange of prune messages
in a network is shown below:
Group membership and network topology can dynamically change
and the prune state of delivery trees should be refreshed at
regular intervals. The RPM algorithm removes the prune
information from routers periodically and the next packet for a
source group member pair is forwarded to all leaf routers. This
is the first drawback of RPM. Relatively big memory space is
required for maintaining static information for all source group
member pairs is another drawback that makes RPM non-scalable
(and therefore, not suitable for very large networks).
Multicast Routing Protocols
This section presents a review of three routing protocols –
Distance Vector Multicast Routing Protocol (DVMRP), Multicast
Extensions to OSPF (MOSPF) protocol, and Protocol
Independent Multicast – Dense Mode (PIM-DM) protocol, which
are more efficient in situations where multicast group members