Abstract : Black holes can occur in packet-switched networks that use distance- vector route calculation algorithms such as tier routing. This section briefly reviews tier routing and defines new terms relevant to black holes. Packet radios use tier routing, a variation on distance-vector routing, to maintain routes to all radios in a non-hierarchical network or to all radios in the same cluster in a hierarchical network. Each radio's routing table contains an entry for each destination packet radio; the entry contains the following information: The destination packet radio ID, The reporting packet radio ID, and Hop count to the destination. Each packet radio periodically broadcasts a Packet Radio Organization Packet (PROP) to all of its neighbor radios, listing its distance to every destination as stored in its routing table. Neighbor packet radios update their routing tables to incorporate shorter routes described within this PROP.