Synchronizing the databases enables the switches to maintain the same view of the topology. PTSPs contain the reachability, link status, and node status information needed to calculate QoS paths. Reachability Information Reachability information is the first step in routing a PNNI request for a connection. The call setup message is directed to a node that advertises a prefix that matches the leading portion of the destination address. The longest matching reachable address prefix is always used.
Reachable addresses can be internal or external.
Internal addresses are known to PNNI to be local, for example, summary end systems attached to the switch. External addresses receive the reachability information from another source. A link to an IISP network is one example of an external address. In addition to topological reachability information, PNNI also advertises detailed information about metrics and attributes for links. Table 1-1 describes these metrics and attributes.