H3C S5500-EI Series Operation Manual page 112

Operation Manual – Multicast
H3C S5500-EI Series Ethernet Switches
Note:
Every activated interface on a router sends hello messages periodically, and thus
learns the PIM neighboring information pertinent to the interface.
II. SPT establishment
The process of building an SPT is the process of "flood and prune".
1) In a PIM-DM domain, when a multicast source S sends multicast data to a
multicast group G, the multicast packet is first flooded throughout the domain: The
router first performs RPF check on the multicast packet. If the packet passes the
RPF check, the router creates an (S, G) entry and forwards the data to all
downstream nodes in the network. In the flooding process, an (S, G) entry is
created on all the routers in the PIM-DM domain.
2) Then, nodes without receivers downstream are pruned: A router having no
receivers downstream sends a prune message to the upstream node to "tell" the
upstream node to delete the corresponding interface from the outgoing interface
list in the (S, G) entry and stop forwarding subsequent packets addressed to that
multicast group down to this node.
Note:
An (S, G) entry contains the multicast source address S, multicast group address G,
outgoing interface list, and incoming interface.
For a given multicast stream, the interface that receives the multicast stream is
referred to as "upstream", and the interfaces that forward the multicast stream are
referred to as "downstream".
A prune process is first initiated by a leaf router. As shown in
any receiver attached to it (the router connected with Host A, for example) sends a
prune message, and this prune process goes on until only necessary branches are left
in the PIM-DM domain. These branches constitute the SPT.
7-3
Chapter 7 PIM Configuration
Figure 7-1, a router without