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Chapter 6 - Protocol Independent Multicast - Dense Mode

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

PIM-DM Packet Forwarding
When a PIM-DM router receives the initial multicast packet from a source, the packet is flooded onto all interfaces in the output interface list (oilist). Recall that the oilist is populated with those interfaces on which neighbors were discovered or on interfaces that have multicast receivers that have indicated their desire to receive the traffic using IGMP. Figure 6-9 shows the various possibilities for forwarding of multicast traffic. Router A has discovered a PIM-DM neighbor on interface S0.
A host has signaled that it wishes to receive multicast traffic for a particular group. The host doesn’t care where the multicast traffic originates, so any packets for this group from any source reaching router A will be forwarded to the host on E0.
Figure 6-9: PIM-DM packet forwarding
No PIM-DM neighbors or multicast receivers have been found on interface S1 so the oilist for this interface will be null. The oilist for the ethernet interface will contain the state (*,G) indicating that router A should forward traffic for group G from any source onto the ethernet interface. The oilist for the S0 interface will contain the state (S,G) indicating that router A should forward multicast traffic for group G from source S to router C. Traffic will also be forwarded if the interface has been manually configured to receive traffic. Traffic is forwarded using the RPF technique, which you will recall, only accepts packets on the interface on the shortest path back to the source. For DVMRP this is generally unambiguous since each DVMRP router runs the same routing protocol. PIM-DM uses whatever IP routing protocol has been configured on the router to determine the RPF technique. We will see how to deal with situations involving a network running more than one IP routing protocol.
Interface States
The oilist for a router interface can be null or in the (*,G) or (S,G) state. An interface can also be in both the (S,G) and (*,G) states. In Figure 6-10, router A has PIM-DM enabled on all interfaces. When the host attaches to the E1 interface of router A, it will join the multicast group 224.0.18.10 by sending an IGMP join message to router A. Router A will add the entry (*,224.0.18.10) to the E1 interface, indicating that multicast traffic for group 224.0.18.10 from any source should be sent onto the ethernet interface. The same (*,G) state can exist in more than one oilist. Input interfaces for a multicast group will have (S,G) state and the same (S,G) state will not exist on more than one interface since a router can only have one best path back to a multicast source. The input interface is the interface over which a router expects to receive multicast traffic from a specific source. This interface is simply the RPF interface.
Figure 6-10: Router state is (*,G) when a receiver joins a multicast group.
In Figure 6-11, router A receives a multicast packet from the source 172.16.1.2 for group 224.0.18.10. Router A creates the (S,G) state for the serial interface since a PIM-DM neighbor has been discovered on this interface. If the serial interface on router A is not on the shortest path back to the source for the downstream router, the interface will be pruned.
Figure 6-11: Routers maintain (S,G) state for multicast sources
In Figure 6-12, we have two sources for the multicast group 224.0.18.10. Router A has a host which has joined this group using IGMP. Router A will accept traffic on interface S1 from the source 172.16.3.2, from router B and on interface S0 from the source 172.16.1.2, and from router C because these are the RPF interfaces for the respective sources. The oilist for the serial interface on router B will contain the (S,G) state (172.16.3.2,224.0.18.10). The serial interface on router C will contain the (S,G) state (172.16.1.2,224.0.18.10).
Figure 6-12: Each multicast source will have (S,G) state on the directly attached router.

 


 
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