Chapter 12. Understanding Protocol Independent Multicast (PIM)

This chapter covers the following key topics about Protocol Independent Multicast (PIM):

• Fundamentals of IGMP version 1, IGMP version 2, and reverse path forwarding (RPF)

• PIM dense mode

• PIM sparse mode

• IGMP and PIM packet format

Host-to-host transmission has been the issue of many discussions in the technical world. As technologies advance, new methodologies for facilitating that transmission emerge. A transmission from one specific host to another specific host is known as a unicast.

One-to-one transmission is easy. The big push, currently, is figuring out how to transmit from one host to many without disrupting traffic flow. Up until now, if you wanted one host to talk to multiple hosts , you had to resort to using a broadcast. Multicast has emerged in recent years as a more efficient alternative.

The difference between multicast, broadcast, and unicast is that unicast packets are destined for one host only. Broadcast packets are destined for all hosts on the same segment, regard-less of whether the host is interested in the packet. Multicast is an efficient method of delivering packets only to hosts interested in the packets. Multicast packets are within the Class D address range of 224.0.0.0 to 239.255.255.255. The multicast sender sends only one copy of the packet, and only the hosts interested in the multicast packet process the packet.

Because multicast packets might traverse several routers before reaching the intended destination, these routers need to have a routing protocol enabled to ensure that multicast packets are delivered efficiently and loop-free .

Several multicast routing protocols have been developed for this matter. One of the first is Distance Vector Multicast Routing Protocol (DVMRP); however, DVMRP is slow in con-vergence and is not scalable. Cisco developed its own multicast routing protocol called Protocol Independent Multicast (PIM). PIM uses the unicast routing table to make forward-ing decisions; therefore, the router's choice of unicast routing protocol could be any of the protocol covered in this book ‚ namely, RIP, IGRP, EIGRP, OSPF, BGP, or IS-IS. PIM works in two different modes ‚ dense mode and sparse mode. Each mode has its own advantages and disadvantages, and each mode has different implementation methods .

Dense mode uses the flood-and-prune mechanism to forward multicast traffic. Dense mode is extremely easy to implement and is less complex than sparse mode; however, dense mode is not scalable in large networks. Therefore, dense mode is more suitable for a small multicast environment.

Sparse mode uses the explicit group -join mechanism to forward multicast traffic. Unlike dense mode, sparse mode is very scalable and can run in a large multicast environment. Because it is more scalable, its implementation is more complex than dense mode, which means that it is harder to troubleshoot.