36 CONFIGURING PIM-SM
This chapter covers the following topics:
■PIM-SM Overview
■PIM-SM Configuration
■Displaying and Debugging PIM-SM
■PIM-SM Configuration Example
■Troubleshooting PIM-SM
PIM-SM Overview Protocol Independent Multicast--Sparse Mode (PIM-SM) is used in the following
conditions:
■The distribution of the group members is relatively separate and the range is
comparatively wide.
■The network bandwidth resource is limited.
PIM-SM is independent of any specific unicast routing protocol. PIM -SM is called
protocol independent because it can use the route information entered by any
routing protocol, such as unicast protocols like OSPF, RIP, or multicasting protocols
like DVRMP in the multicasting routing information base (RIB). It supposes that all
the routers will not send multicast packets to the multicast group unless there is an
explicit transmission request. PIM-SM informs all the PIM-SM routers of multicast
information by configuring a rendezvous point (RP) and a bootstrap router (BSR).
And it reduces data messages and controls the network bandwidth occupied by
the messages occupy by allowing routers to explicitly join and leave multicast
groups. PIM-SM constructs an RP path tree (RPT) with the RP its root so as to make
the multicast packets transmitted along with the RPT.
When a host joins a multicast group, the directly connected router sends a joining
message to the RP PIM. The first hop router of the transmitter registers the
transmitter at RP. The receiver's DR adds the receiver to the RPT. Using the RPT
with the RP its root not only reduces the protocol state that routers need
maintenance, which improves the scalability of the protocol and reduces the
router's processing cost, but also supports a large number of simultaneous
multicast groups. When the data traffic flow reaches a certain degree, the data
will switch from the RPT to the shortest path tree based on source so as to reduce
the network delay.
PIM-SM mainly includes the following kinds of messages:
■PIM Hello Message: A PIM hello message is periodically sent to the other
neighbor interface by a router interface that operates PIM-DM protocol in the
same network segment. It establishes neighborship with the PIM-DM