H3C S7500 Series Operation Manual page 476

Operation Manual – Multicast
H3C S7500 Series Ethernet Switches
Neighbor discovery
SPT establishing
Graft
RPF check
Assert mechanism
I. Neighbor discovery
In a 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.
II. 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 a (S, G) entry 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 the PIM-DM
domain.
If there is no multicast group member in the downstream nodes, the router will send a
prune message to the upstream node to inform them not to forward data to the branch
any more. The upstream node, 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 left in the PIM-DM
domain. In this way, an SPT rooted at source S is established.
The pruning process is initiated by leaf routers. As shown in
without receivers (such as the router connected to Host A) initiate the pruning process
actively.
7-2
Chapter 7 PIM Configuration
Figure 7-1, the routers