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Reference Guide for the Centillion 50/100 and Model 5000BH Switches
4-10
893-01006-C
One good reason for mixing PNNI and IISP is to prevent routing tables from
becoming unwieldy. It also prevents unnecessary propagation of topology changes
that are not relevant to distant switches. Another reason for mixing PNNI and IISP
is that PNNI enables the switch to do more intelligent call balancing across
multiple links, which should result in greater efficiency and improved behavior
during recovery from a failure.
One configuration is to use IISP for the links between the transit switches and the
core. If the ATM network is expected to expand, either in terms of the number of
switches in a location or by interconnecting remote locations, it is best if the core
is a PNNI group even if it consists of only a pair of switches. This configuration
ensures that, as the network expands, the correct addressing will be used.
For small and medium-size networks, it will be unnecessary to have PNNI at the
edge and PNNI in the core; and, in this case, either the edge (wiring closet) or the
core (data center) should be solely IISP. There are advantages and disadvantages
to both of the approaches.
Using PNNI in the edges or wiring closets with IISP in the core or data center has
the advantage of better flexibility and handling of changes. The majority of
network changes take place in the edge where switches and links are added as
traffic requires, whereas the core remains static. With PNNI in the edge, these
changes require little or no reconfiguration of the switches involved.
Using PNNI at the core or data center and IISP at the edge or wiring closet devices
has the following advantages:
• The edge or wiring closet devices require the least configuration. Route tables
for these devices need only include route entries with a default route for both
uplinks.
• The core switch requires a more complex configuration to learn the network
topology and configure appropriate routes and alternate routes as needed.
The core switches must have IISP routes with the proper configuration.
PNNI then advertises this information to the PNNI cloud.