Unspecified Bit Rate
IBM's PNNI Path Selection supports Unspecified Bit Rate (UBR) in two ways, shortest path and widest
path:
The shortest path approach follows a two step algorithm. In step one, paths with minimal hop count to
the destination are selected. In the second step, the widest path approach is applied to the previously
selected group of shortest paths to select the final route. This approach is favored when the network
contains critical restraints such as links (VCIs, VPIs) and/or switches that tend to become traffic
bottlenecks. The drawback of the shortest path approach, is its reduced load balancing capability.
The widest path approach finds the least loaded path in terms of bandwidth regardless of the number
of hops required to reach the destination. This approach balances the load on the paths through a
network in the absence of critical constraints within that network.
Regardless of the criterion used, UBR path computation is always done in precomputed mode.
The default configured setting is the widest path approach, and this can be changed to shortest path by
entering the following command:
à
ð
8265ATM
>
set pnni path_selection ubr: shortest_path
á
ñ
The setting can be changed back to widest path by entering the following command:
à
ð
8265ATM
>
set pnni path_selection ubr: widest_path
á
ñ
To display the current route modes, enter the following command:
à
ð
8265ATM
>
show pnni path_selection
á
ñ
Note: Point-to-multipoint calls are processed as on-demand, shortest path.
Chapter 6. Configuring PNNI Parameters
67
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