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Chapter 6 - Protocol Independent Multicast - Dense Mode

Cisco Multicast Routing & Switching
William R. Parkhurst
  Copyright © 1999 The McGraw-Hill Companies

Chapter 6: Protocol Independent Multicast — Dense Mode
Overview
Protocol Independent Multicast-Dense Mode (PIM-DM) is similar to Distance Vector Multicast Routing Protocol (DVMRP) in a number of ways. Both are referred to as dense mode protocols. A dense mode protocol operates in an environment where the multicast sources and multicast receivers are located in the same area, such as a local area network (LAN). Dense mode protocols also assume that bandwidth is not a limiting factor. Both protocols operate using a broadcast and prune methodology where multicast routers assume everyone wants to receive multicast traffic. Under this model, traffic from a multicast source is sent on all downstream interfaces until an interface is pruned from the multicast tree. An interface has a limited prune time after which the interface is grafted back onto the multicast delivery tree and multicast traffic is again flooded onto the network. Both protocols create source-based delivery trees that connect each specific multicast source with each downstream receiver. Source trees are dynamically created for each source using the Reverse Path Forwarding (RPF) technique. The major difference between DVMRP and PIM-DM is that DVMRP uses a built-in multicast routing protocol while PIM-DM relies on the configured unicast routing protocol. This means that you can use any of the IP routing protocols (RIP, IGRP, EIGRP, or OSPF) with PIM-DM.
PIM-DM is independent of the IP routing protocol chosen to run on your network, hence the name, Protocol Independent Multicast. This also means that in the same network DVMRP and PIM could possibly construct divergent source based delivery trees as shown in Figures 6-1, 6-2, and 6-3. In Figure 6-1, DVMRP is being used as the multicast routing protocol. Since DVMRP builds routing tables based on RIP, the source based tree for the network in Figure 6-1 would be through the 28.8K connections since this path offers a lower hop count than the path through the T1 connections.
Figure 6-1: DVMRP source-based tree
In Figure 6-2, OSPF is the unicast routing protocol which has a metric based on the link speed and not the hop count. In this case, the shortest path from the receiver to the source is through the T1 connections instead of the 28.8K connections. Figure 6-3 shows that PIM-DM is independent of the unicast routing protocol in the sense that it doesn’t matter which unicast routing protocol is used since PIM-DM will still operate. Figure 6-3 does show that PIM-DM is, in some ways actually, dependent on the selected unicast routing protocol since the source based delivery tree can be different depending on the protocol used.
Figure 6-2: PIM-DM source-based tree in an OSPF environment
Figure 6-3: PIM-DM source-based tree in an RIP environment

 


 
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