3Com 10014299 manual PIM-SM Overview

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