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Configuring PIM
This chapter describes PIM, how to configure PIM, configuration examples, and troubleshooting
methods.
Overview
PIM provides IP multicast forwarding by leveraging unicast static routes or unicast routing tables
generated by any unicast routing protocol, such as RIP, OSPF, IS-IS, or BGP. Independent of the
unicast routing protocols running on the device, multicast routing can be implemented as long as the
corresponding multicast routing entries are created through unicast routes. PIM uses the RPF
mechanism to implement multicast forwarding. When a multicast packet arrives on an interface of
the device, it undergoes an RPF check. If the RPF check succeeds, the device creates the
corresponding routing entry and forwards the packet. If the RPF check fails, the device discards the
packet. For more information about RPF, see "
Configuring multicast routing and forwarding
."
Based on the implementation mechanism, PIM falls into the following categories:
•
Protocol Independent Multicast–Dense Mode (PIM-DM)
•
Protocol Independent Multicast–Sparse Mode (PIM-SM)
•
Bidirectional Protocol Independent Multicast (BIDIR-PIM)
•
Protocol Independent Multicast Source-Specific Multicast (PIM-SSM)
The term "interface" in this chapter collectively refers to
Layer 3 interfaces, including
VLAN interfaces
and Layer 3 Ethernet interfaces. You can set an Ethernet port as a Layer 3 interface by using the
port link-mode route
command (see
Layer 2—LAN Switching Configuration Guide
).
PIM-DM
PIM-DM is a type of dense mode multicast protocol. It uses the push mode for multicast forwarding,
and is suitable for small-sized networks with densely distributed multicast members.
The following describes the basic implementation of PIM-DM:
•
PIM-DM assumes that at least one multicast group member exists on each subnet of a network.
Therefore, multicast data is flooded to all nodes on the network. Then, branches without
multicast forwarding are pruned from the forwarding tree, leaving only those branches that
contain receivers. This flood-and-prune process takes place periodically. Pruned branches
resume multicast forwarding when the pruned state times out. Data is flooded again down these
branches, and then the branches are pruned again.
•
The graft mechanism is used to reduce the latency for resuming the forwarding capability of a
previously pruned branch.
Generally speaking, the multicast forwarding path is a source tree. That is, it is a forwarding tree with
the multicast source as its "root" and multicast group members as its "leaves." Because the source
tree is the shortest path from the multicast source to the receivers, it is also called a SPT.
The operating mechanism of PIM-DM is summarized as follows:
•
•
•
•
