IBM 8265 Nways ATM Switch, Руководство пользователя, Страница: 22 / 192

PNNI Routing
IBM's PNNI supports three types of path selection:
Ÿ Constant Bit Rate (CBR), real time Variable Bit Rate (rt VBR), and non-real time Variable Bit Rate (nrt
VBR).
Routing is done on demand:
– Calls not satisfying the Generic Call Admission Control (GCAC) are pruned.
– A shortest path is computed based on the Administrative Weight.
Ÿ Available Bit Rate (ABR)
There are two types of ABR path selection, precomputed and on-demand:
– With precomputed path selection, the specific route is obtained via table look-ups, resulting in fast
connection setup.
– With on-demand path selection, more optimization for the individual routes is possible, but
connection setup is slower.
Note: When MCR=0, the ABR is treated the same as UBR.
Ÿ Unspecified Bit Rate (UBR).
There are two types of UBR path selection, widest path and shortest path:
– 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.
– The shortest path approach follows a three step algorithm.
1. In the first step, path selection is based on the administrative weight.
2. In the second step, paths with minimal hop count to the destination are selected.
3. In the third step, the widest path approach is applied to the previously selected group of
shortest paths to select the final route.
The shortest path 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.
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IBM 8265: User's Guide