Hello Michael
Oversubscribing refers to the practice of designing a network, or arranging network traffic patterns, in such a way as to end up having more traffic arrive at an interface than it is able to carry. In other words, if you oversubscribe a particular network path, or interface, you are essentially creating a bottleneck.
For example, take a look at this diagram from the lesson:
Imagine we have traffic arriving from R3 and from R1, being routed towards R2. Note that all of the router interfaces are FastEthernet 100Mbps interfaces. Imagine that at a particular point in time, R3 is sending 60 Mbps to R2, and R1 is sending 80 Mbps to R2. Because R2’s interface can handle only up to 100Mbps, this interface is considered oversubscribed.
Network traffic patterns, as well as general network architecture must be designed in such a way as to minimize these types of oversubscriptions. Sometimes, however, they do occur, even in the best network design, because user traffic patterns cannot always be successfully forecasted.
This is where the DSBM feature becomes useful, as it creates a centralized location where all RSVP information is kept within a particular network segment. The DSBM can manage the bandwidth within the segment appropriately to avoid the oversubscription of any single interface.
I hope this has been helpful!
Laz