Cisco Systems Network Router manual How MGX and SES Nodes Select Links, On-Demand Routing

Chapter 4 Planning Intermediate Route Selection

How MGX and SES Nodes Select Links

Any route selected from the SPTs must conform to all specified metrics. If a conforming route is not available in chosen SPT, on demand routing is used to find a conforming route.

On PXM1E cards and service modules, you can change this with the addcon command.

On-Demand Routing

When the SPTs cannot produce a route for a connection, the switch performs on-demand routing. A SPT can fail to produce a route because the shortest route or routes in the table have failed. On-demand routing is also required when a connection specifies multiple routing metrics and the SPT routes do not conform to all of the metrics.

During on-demand routing, the switch searches the PNNI database for routes that match the specified criteria. On demand routing takes more time than SPT routing. However, on-demand routing can access more of the PNNI database and select better routes.

As a switch administrator, you can choose what action the controller takes when it discovers the first acceptable on-demand route. You can configure the controller for first fit, which produces the fastest route selection, or you can configure the switch for best fit. When the controller is configured for first fit on-demand route selection, it selects the first route that satisfies all connection requirements. When the controller is configured for best fit on-demand route selection, it identifies all routes that meet the call requirements, and then it chooses the route based on the setting of the load balancing option.

Load Balancing for SPT and On-Demand Routing

The load balancing option, which applies to SPT routing and on-demand routing, is a configurable parameter that allows you to control how a route is chosen when multiple routes offer the same level of service. You can configure the load balancing option to choose randomly from multiple routes or choose according to the best AvCR. If you select the random method, the PNNI controller considers the conforming routes equal and balances the load by randomly assigning calls to each. If you choose the route based on the AvCR, the route with the highest available cell rate is chosen.

How MGX and SES Nodes Select Links

The SPTs are built by calculating routes that are optimized for lowest AW, CTD, or CDV. However, for most service classes, each link along a route must conform to additional parameters. If no route is found in the SPTs, on-demand routing must be used to calculate a conforming route from the PNNI database.

The link parameter requirements for a service class establishes the quality of service (QoS) required for a call. Table 4-3shows the link parameters that must be satisfied for each service class.

	Cisco PNNI Network Planning Guide for MGX and SES Products, Release 5
4-6	Part Number OL-3847-01 Rev. D0, April, 2004