F-8ForeThought PNNIF.2.1 Peer GroupsThe FT-PNNI hierarchy begins with a network of switches, organized intopeer groups. A peer group is a collection of interconnected switches that areorganized into a group. Peer group organization can be determined by a net-work administrator, but switches that are located close to one another areusually made into a peer group.The network shown in Figure F.3 is organized into four lowest-level peergroups: A, B, C, and D. The switches within a certain peer group are num-bered according to that particular group. For example, the switches in peergroup A are identified as A.1, A.2, and A.3.Peer groups have a peer group identifier (ID), assigned at configuration timeand exchanged in hello indication messages. Switches can determine in whichpeer group they are located by comparing these peer group IDs.Switches in a peer group are aware of the topology of their own peer groupand the existence of all other peer groups. They recognize the links betweentheir peer group and others, but they are not aware of internal topology infor-mation of other peer groups. Instead, the topologies of other peer groups aresummarized as a single, reachable location, known as a peer group summarynode (PGSN).F.2.2 Peer Group TopologyTo maintain an accurate and updated view of its relative location and status, aswitch sends hello indication messages to every other switch with which ithas a direct connection. These hello indications contain the switch prefix, peergroup membership information, and link metrics (attributes) for the physicallink between the two switches.Through this exchange of messages, each switch learns which switches are itsimmediate neighbors, to what peer groups they belong, and whether or notthe link between itself and its neighbors is valid.F.2.3 Border SwitchesA border switch is any switch that has at least one link to a switch in anotherpeer group. Border switches play an important role in FT-PNNI because theyare responsible for summarizing reachability information for their respectivepeer groups, appropriately filtering the flow of topology database informa-tion across peer group boundaries, and building the lowest level source routefor call setups entering the peer group.
