The fat tree is a network topology well suited for use as the interconnection network in systems such as parallel computers. Its large number of paths between every source/destination pair gives the fat tree the ability to provide high throughput. This also gives it a high probability of tolerating network faults statically, but few algorithms to dynamically tolerate faults in fattrees have previously been proposed. In this paper we present a deadlock free routing method for providing dynamic fault tolerance through misrouting downwards in the network. We show that the algorithm is one fault-tolerant, and that it with a certain probability can tolerate a large number of faults.