Introduction to Protocol Independent Multicast (PIM)

If you are a CCIE candidate, studying the previous sections on protocols not supported or only partially supported (in the case of DVMRP) by Cisco may strike you as a poor investment of time. Yet each protocol offers lessons in what is desirable about a multicast routing protocol and what is not.

DVMRP shares the characteristic of unicast distance vector protocols of being very simple to implement ”little more is required than to just turn it on. But this simplicity comes at the expense of high overhead, creating serious scaling problems in anything other than small, high-bandwidth networks densely populated with group members .

MOSPF brings its link-state advantages to the table, but at the cost of increased design complexity. Its use of explicit joins eliminates DVMRP's topsy-turvy rule that routers not forwarding for a particular group must remember (hold state) that they are not forwarding packets for that group. The result is a reduced impact on network resources. Yet MOSPF's source-based trees still make the protocol unsuitable for topologies sparsely populated with group members. Given the limited increase in scalability, many, if not most, network designers are unwilling to pay the cost of MOSPF's more-complex topological requirements.

DVMRP is "self-contained," in that it uses its own built-in protocol to locate the unicast addresses necessary for the creation and maintenance of multicast trees. In this sense it is completely independent of any underlying unicast routing protocol, but the price of this independence is the consumption of network resources to gather information that probably already exists in the unicast routing table.

NOTE

This cost is not as high as it might seem. As the section "PIM-DM Basics" explains, costs also are associated with running a flood-and-prune protocol without a built-in unicast component.

MOSPF, on the other hand, is a multicast extension of a unicast protocol. So while MOSPF eliminates the redundancy of a separate unicast protocol, it cannot run independently of OSPF.

CBT introduces true protocol independence. It consults the existing unicast routing table for unicast destinations, without regard for what protocol is used to maintain that table. CBT also is scalable to sparse topologies, although core placement must be carefully planned to minimize suboptimal paths and traffic bottlenecks. At this time, CBT is stuck in a Catch-22: The interest in the protocol for real-world applications is limited by its lack of maturity, and the protocol lacks maturity because of its limited use in the real world. CBT is unlikely to move into mainstream acceptance unless and until its designers can introduce significant advantages over the currently favored and more versatile PIM-SM.

PIM is the only IP multicast routing protocol fully supported by Cisco IOS. (DVMRP is supported only to the degree that PIM can connect to a DVMRP network.)

Like CBT, and as its name asserts, PIM is protocol-independent . That is, it uses the unicast routing table to locate unicast addresses, without regard for how the table learned the addresses.

There is a standard list of PIM message formats. Some messages are used only by PIM-DM, some are used only by PIM-SM, and some are shared. All message formats, including those used only by PIM-DM, are described at the end of the section "Protocol Independent Multicast, Sparse Mode (PIM-SM)."

The current version of PIM is PIMv2. Version 1 of the protocol encapsulates its messages in IP packets with protocol number 2 (IGMP) and uses the multicast address 224.0.0.2. PIMv2, which is supported beginning with Cisco IOS Software Release 11.3(2)T, uses its own protocol number of 103 and the reserved multicast address 224.0.0.13. When a PIMv2 router peers with a PIMv1 router, it automatically sets that interface to PIMv1.