3com 8807 사용자 가이드

PIM-SM (Protocol Independent Multicast, Sparse Mode) belongs to sparse mode 
multicast routing protocols. PIM-SM is mainly applicable to large-scale networks 
with broad scope in which group members are relatively sparse.

Different from the flood & prune principle of the dense mode, PIM-SM assumes 
that all hosts do not need to receive multicast packets, unless there is an explicit 
request for the packets.

PIM-SM uses the RP (Rendezvous Point) and the BSR (Bootstrap Router) to 
advertise multicast information to all PIM-SM routers and uses the join/prune 
information of the router to build the RP-rooted shared tree (RPT), thereby 
reducing the bandwidth occupied by data packets and control packets and 
reducing the process overhead of the router. Multicast data flows along the shared 
tree to the network segments the multicast group members are on. When the 
data traffic is sufficient, the multicast data flow can switch over to the SPT 
(Shortest Path Tree) rooted on the source to reduce network delay. PIM-SM does 
not depend on the specified unicast routing protocol but uses the present unicast 
routing table to perform the RPF check.

Note that, the creation and interaction of the RPs and BSRs are implemented 
through periodical RP advertisements and BSR Bootstrap packets respectively.

To make PIM-SM operate, you must configure candidate RPs and BSRs. BSRs 
collect and broadcast the information from candidate RPs.

The PIM-SM working process is as follows: neighbor discovery, building the 
RP-rooted shared tree (RPT), multicast source registration and SPT switchover etc. 
The neighbor discovery mechanism is the same as that of PIM-DM, which will not 
be described any more.

When hosts join a multicast group G, the leaf routers that directly connect with 
the hosts send IGMP messages to learn the receivers of multicast group G. In this 
way, the leaf routers calculate the corresponding rendezvous point (RP) for 
multicast group G and then send join messages to the node of the next level 
toward the rendezvous point (RP).

Each router along the path between the leaf routers and the RP will generate (*, 
G) entries in the forwarding table, indicating that all packets sent to multicast 
group G are applicable to the entries no matter from which source they are sent. 
When the RP receives the packets sent to multicast group G, the packets will be