[Contents] [Prev] [Next] [Index] [Report an Error]

Interface Lists

To avoid multicast routing loops, every multicast router must always be aware of the interface that leads to the source of that multicast group content by the shortest path. This is the upstream (incoming) interface, and packets should never be forwarded back toward a multicast source. All other interfaces are potential downstream (outgoing) interfaces, depending on the number of branches on the distribution tree.

Routers closely monitor the status of the incoming and outgoing interfaces, a process that determines the multicast forwarding state. A router with a multicast forwarding state for a particular multicast group is essentially “turned on” for that group's content. Interfaces on the router's outgoing interface list send copies of the group's packets received on the incoming interface list for that group. The incoming and outgoing interface lists might be different for different multicast groups.

The multicast forwarding state in a router is usually written in either (S,G) or (*,G) notation. These are pronounced “ess comma gee” and “star comma gee,” respectively. In (S,G), the S refers to the unicast IP address of the source for the multicast traffic, and the G refers to the particular multicast group IP address for which S is the source. All multicast packets sent from this source have S as the source address and G as the destination address.

The asterisk (*) in the (*,G) notation is a wildcard indicating that the state applies to any multicast application source sending to group G. So, if two sources are originating exactly the same content for multicast group 224.1.1.2, a router could use (*,224.1.1.2) to represent the state of a router forwarding traffic from both sources to the group.

For more information about the use of multicast forwarding state notations in different types of distribution trees, see Rendezvous Point, Shared Trees, and the Rendezvous-Point Tree. For more information about the use of multicast notations in different multicast routing protocols, see Protocols for Multicast.

[Contents] [Prev] [Next] [Index] [Report an Error]