IGMP Snooping internal interface
In a Multicast topology, when implementing IGMP snooping, the switch involved in the process uses what is referred to as an internal interface as part of the process. The internal interface is simply a logical construct used within the context of IGMP snooping to describe how the CPU learns about MAC addresses and the interfaces to which they correspond so that it can populate the MAC address table or CAM table. Take a look at the following diagram:
The internal interface is depicted by the CPU. You can see the INT interface included in the CAM table.