3Com Switch 7750 Configuration Guide Guide

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PIM CONFIGURATION
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 only if
unicast routing protocols can generate route entries.
With the help of the reverse path forwarding, 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.
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 network are:
■ Members in a multicast group are dense.
■ 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 points 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 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.
Work Mechanism of
PIM-DM
The working procedure of PIM-DM is summarized as follows:
■ Neighbor discovery
■ SPT establishing
■ Graft