Could someone please explain to me how to derive the forward equations for the following question:

Customers arrive at a queue according to a poisson process with intensity lamba. Most of the time, there is no server but every now and then, there appears a super server which serves all the customers in the queue instantaneously, emptying the queue. Time intervals between the successive appearances of a super server are independent and exponentially distributed with parameter u. Initially, the queue is empty. Let Pn(t) denote the probability that there are n customers in the queue at time t.

Thanks in advance!