Operation Manual – Multicast
H3C S3600 Series Ethernet Switches-Release 1510 Chapter 7 PIM Configuration
7-1
Chapter 7 PIM Configuration
7.1 PIM Overview
Protocol independent multicast (PIM) means that the unicast routing protocols
providing routes for the multicast could be static routes, RIP, OSPF, IS-IS, or BGP. The
multicast routing protocol is independent of unicast routing protocols as long as unicast
routing protocols can generate route entries.
With the help of reverse path forwarding, PIM can transmit multicast information in the
network. For the convenience of description, a network consisting of PIM-enabled
multicast routers is called a 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:
z Members in a multicast group are dense.
z PIM-DM assumes that in each subnet of the network there is at least one receiver
interested in the multicast source.
z Multicast packets are flooded to all the nodes in the network, and the related
resources (such as 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
that do not forward multicast data and keeps only the branches containing receivers. In
order that the pruned branches that are demanded to forward multicast data can
receive multicast data flows again, the pruned braches can be restored to the
forwarding status periodically.
In order to reduce the delay time for a pruned branch to be restored to the forwarding
status, PIM-DM uses the graft mechanism to restore the 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.
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