Chapter 7 Pim Configuration; Pim Overview; Introduction To Pim-Dm; Work Mechanism Of Pim-Dm - H3C S7500 Series Operation Manual

Operation Manual – Multicast
H3C S7500 Series Ethernet Switches

7.1 PIM Overview

Protocol independent multicast (PIM) means that the unicast routing protocols
providing routes for IP multicast could be static routes, RIP, OSPF, IS-IS, or BGP. The
multicast routing protocol is independent of unicast routing protocols only if unicast
routing protocols can generate route entries.
With the help of the reverse path forwarding (RPF), PIM can transmit multicast
information in the network. For the convenience of description, the network consisted of
PIM-enabled multicast routers is called PIM multicast domain.

7.1.1 Introduction to PIM-DM

Protocol independent multicast dense mode (PIM-DM) is a dense mode multicast
protocol. It is suitable for small networks.
The features of such networks are:
Members in a multicast group are densely distributed.
PIM-DM assumes that in each subnet of the network there is at least one receiver
interested in the multicast source.
Multicast packets are flooded to all the nodes in the network, and the related
resources (bandwidth and the CPU of the router) are consumed at the same time.
In order to reduce the network resource consumption, PIM-DM prunes the branches
which do not forward multicast data and keeps only the branches including receivers. In
order that the pruned branches which are demanded to forward multicast data can
receive multicast data flows again, the pruned branches can be restored to the
forwarding status periodically.
In order to reduce the delay time for a pruned branch to resume the forwarding status,
PIM-DM uses the graft mechanism to resume multicast packet forwarding automatically.
Such periodical floods and prunes are the features of PIM-DM, which is suitable for
small LANs only. The flood-prune technology adopted in PIM-DM is unacceptable in
WAN.
Generally, the packet forwarding path in PIM-DM is a shortest path tree (SPT) with the
multicast source as the root and multicast members as the leaves. The SPT uses the
shortest path from the multicast source to the receiver.

7.1.2 Work Mechanism of PIM-DM

The working procedure of PIM-DM is summarized as follows:

Chapter 7 PIM Configuration

7-1
Chapter 7 PIM Configuration