Chapter 28 Pim-Dm Configuration; Pim-Dm Overview; Introduction To Pim-Dm; Pim-Dm Working Principle - 3Com 8800 Configuration Manual

3Com Switch 8800 Configuration Guide

Chapter 28 PIM-DM Configuration

28.1 PIM-DM Overview

28.1.1 Introduction to PIM-DM

PIM-DM (Protocol Independent Multicast, Dense Mode) belongs to dense mode
multicast routing protocols. PIM-DM is suitable for small networks. Members of
multicast groups are relatively dense in such network environments.

28.1.2 PIM-DM Working Principle

The working procedures of PIM-DM include neighbor discovery, flood & prune and
graft.
I. Neighbor discovery
The PIM-DM router needs to use Hello messages to perform neighbor discovery when
it is started. All network nodes running PIM-DM keep in touch with one another with
Hello messages, which are sent periodically.
II. Flood&Prune
PIM-DM assumes that all hosts on the network are ready to receive multicast data.
When a multicast source "S" begins to send data to a multicast group "G", after the
router receives the multicast packets, the router will perform RPF check according to
the unicast routing table first. If the RPF check is passed, the router will create an (S, G)
entry and then flood the data to all downstream PIM-DM nodes. If the RPF check is not
passed, that is, multicast packets enter from an error interface, the packets will be
discarded. After this process, an (S, G) entry will be created in the PIM-DM multicast
domain.
If the downstream node has no multicast group members, it will send a Prune message
to the upstream nodes to inform the upstream node not to forward data to the
downstream node. Receiving the prune message, the upstream node will remove the
corresponding 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 leaf routers first.
This process is called "flood & prune" process. In addition, nodes that are pruned
provide timeout mechanism. Each router re-starts the "flood & prune" process upon
pruning timeout. The consistent "flood & prune" process of PIM-DM is performed
periodically.
Chapter 28 PIM-DM Configuration
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