PNNI(Private network-network interface)

PNNI provides two significant services:
ATM topology discovery and call establishment. For switches to build connections between end points, the switch must know the ATM network topology. PNNI is the ATM routing protocol that enables switches to automatically discover the topology and the characteristics of the links interconnecting the switches. A link-state protocol much like OSPF, PNNI tracks things such as bandwidth on links. When a significant event occurs that changes the characteristics of a link, PNNI announces the change to the other switches.

When a station sends a call setup request to its local switch, the ingress switch references the PNNI routing table to determine a path between the source and the intended destination that meets the QoS requirements specified by the source. The switch attached to the source then builds a list defining each switch hop to support the circuit to the destination. This is called the designated transit list (DTL).

VCI = 18 is reserved for PNNI.