Cabletron Systems MMAC-Plus SFCS-1000, Configuration Manual

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F-5
ForeThought PNNI
F.2 The Physical Network
In an ATM network, data is sent and received over virtual circuits, or circuits
that only exist when needed. This communication over these virtual circuits is
made possible by signalling that occurs between the switches in the network.
In a network of Cabletron switches, any new addition to the topology is rec-
ognized immediately by all nodes (switches) having a direct connection to the
new node. Then each switch that has recognized the new switch sends a mes-
sage to each of its direct connections, and so on. Eventually, and within a very
brief period of time, every switch in the network is aware of the new addition
and the links by which that new addition can be reached. This topology is
stored by each switch in a topology database.
In a small, local area network (LAN), the topology is relatively simple, mean-
ing that the switches in the LAN have a relatively small topology database to
maintain. As the LAN grows, however, and more switches are added, that
same database can grow to be very large in a short period of time.
As this topology database grows, the amount of information a switch must
consult when searching for an address also grows. In the end, this can result
in delayed connection set-up, congestion in the network, and even lost data.
Figure F.2 depicts a typical ATM network, containing 21 Cabletron switches
(
). The hierarchy of this network is flat, meaning that each switch must be
aware of all the other switches in the network, as well as all the possible
routes to those switches. As more switches are added to this network, the
hierarchy will become more complex and the switches will have to contend
with larger topology databases.