: The Private Network Node Interface (PNNI), standardized by the ATM Forum (see [1]), provides a flexible and scaleable routing architecture for ATM networks comprising a routing protocol and a signaling protocol. In PNNI networks PTSEs are used to distribute topology information through the network. This paper investigates the PTSE rate in PNNI nodes due to flooding. Beside theoretical results, the paper also presents some measurements of example networks.