3Com Switch 7750 Configuration Guide

462 CHAPTER 47: PIM CONFIGURATION
■ RPF check
■ Assert mechanism
Neighbor discovery
In PIM-DM network, the multicast router needs to use Hello messages to perform
neighbor discovery and maintain the neighbor relation when it is started. All
routers keep in touch with each other through sending Hello messages
periodically, and thus SPT is established and maintained.
SPT establishment
The procedure of establishing SPT is also called Flooding&Prune.
The procedure is as follows:
■ PIM-DM assumes that all hosts on the network are ready to receive multicast
data.
■ When a multicast router receives a multicast packet from a multicast source
“S” to a multicast group “G”, it begins with RPF check according to the unicast
routing table.
■ If the RPF check passes, the router will create an entry(S, G) and forward the
packet to all the downstream PIM-DM nodes. That is the process of flooding.
■ If not, that is, the router considers that the multicast packets travel into the
router through incorrect interfaces, the router just discards the packets.
After this process, the router will create a (S, G) entry for every host in PIM-DM
domain.
If there is no multicast group member in the downstream nodes, the router will
send a prune message to the upstream nodes to inform them not to forward data
any more. The upstream nodes, as informed, will remove the relative interface
from the outgoing interface list corresponding to the multicast forwarding entry
(S, G). The pruning process continues until there are only necessary branches in
PIM-DM. In this way, a SPT (Shortest Path Tree) rooted at source S is established.
The pruning process is initiated by leaf routers. As shown in Figure 112, the
routers without receivers (such as the router connected to User A) initiates the
pruning process automatically.