Chapter 6 Pim Configuration; Pim Overview; Pim-Dm - Huawei Quidway S6500 Series Operation Manual

Operation Manual - Multicast Protocol
Quidway S6500 Series Ethernet Switches

Chapter 6 PIM Configuration

6.1 PIM Overview

6.1.1 PIM-DM

I. Introduction
Protocol independent multicast-dense mode (PIM-DM) is the dense-mode multicast
routing protocol. It is applicable to small networks, where multicast group members are
rather intensive.
II. Working principle
The operation of PIM-DM includes neighbor discovery, flooding and pruning, and
grafting.
1) Neighbor discovery
The PIM-DM router discovers neighbors by regularly sending Hello message when it
starts.
2) Flooding and Pruning
PIM-DM assumes that all hosts on the network are ready to receive multicast data.
When a router receives a multicast packet from a multicast source "S" to a multicast
group "G", it begins with RPF check by 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, 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). In this way, a SPT
(Shortest Path Tree) rooted at source S is built. The pruning process is initiated by
routers.
The above process is called "Flooding and Pruning". Every pruned node also provides
timeout mechanism. If pruning behavior times out, the router shall initiate another
flooding and pruning process. This process shall continue periodically for PIM-DM.
PIM-DM builds upon the present unicast routing table a data source-rooted multicast
forwarding tree to conduct RPF check. When a multicast packet arrives, the router first
checks the path. If the interface this packet reaches is the one along the unicast route
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Chapter 6 PIM Configuration