Interface States

Nov 24,2010 by alperen

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).