F-2ForeThought PNNIF.1 FT-PNNI RoutingThe FT-PNNI routing protocol serves to distribute topology and addressreachability information between switches and groups of switches in a net-work. This topology and addressing information is used by switches to com-pute paths through the network. FT-PNNI routing has the following aspects:• Hello Protocol• Topology Database Exchange• Flooding• Hierarchical RoutingF.1.1 Hello ProtocolAt regular intervals, each switch transmits a hello indication on each of itsFT-PNNI routing channels. The time between these hello indications is calledthe Hello Indication Interval. When a switch receives a hello indication fromone of its neighbors, it initializes the logical link (loglink) from that neighborto itself in the topology database.Since hello indications are sent periodically, loglinks are also refreshed peri-odically. Note that loglinks discovered as a result of hello indications are uni-directional. Each switch initializes unidirectional loglinks that have anadjacent switch as the destination.F.1.2 Topology Database ExchangeAt regular intervals, called the NSAP map indication interval, each switchsends to each of its neighbors a group of loglinks taken from its topologydatabase. This is done to ensure that the topology databases of the switchesstay synchronized.F.1.3 FloodingWhenever a new loglink is discovered by a switch, the switch immediatelybroadcasts the existence of this link to all of its neighbors. The neighboringswitches then broadcast the existence of the same link to all of their neigh-bors. Very quickly, the existence of the new loglink is flooded immediately toall of the switches in the network.Similarly, when a link goes down, that information is propagated through thenetwork immediately. Also, when a significant change is seen in the metrics ofa loglink, the latest state of the loglink is propagated immediately throughoutthe network.
