PIM-DM Packet Forwarding

Nov 24,2010 by alperen

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.

PIM-DM Packet Forwarding

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