Cisco Systems Network Router manual Planning Intermediate Route Selection, Link and Route Metrics

C H A P T E R 4

Planning Intermediate Route Selection

When a PNNI network node receives a call request, there can be multiple routes available that meet the quality of service (QoS) requirements for the call. This chapter describes how PNNI selects a route from multiple acceptable routes, and it describes parameters that you can modify to control route selection.

How MGX and SES Nodes Select Routes

MGX and SES nodes provide for the following PNNI route sources:

•Manually defined preferred routes (Release 3 and later)

•Pre-calculated routing tables called shortest path tables

•On-demand routes calculated from PNNI database entries

The following sections describe the link and route metrics used during routing, how shortest path table routing works, and how on-demand routing works. At the end of this chapter is a section on additional routing feature provided by MGX and SES nodes.

Link and Route Metrics

Most route metrics are calculated based on the link metrics for each link along the route. Because of this, link and route metrics often use the same name or similar names. This can be confusing if you do not consider the context in which the terms are used. Link metrics apply when configuring an individual link or when choosing between two or more links. Route metrics apply when configuring a connection or choosing between two or more routes.

The following sections introduce some of the most common link and route metrics and explain the differences between their use as either link or route metrics.

Administrative Weight

Administrative weight (AW) is a configurable cost that can be defined for each link in a PNNI network. The default link AW is 5040. There is no significance to the cost units. What is important is how the cost relates to that for other links in the network. For example, if two parallel links between two nodes have different costs, and if the link selection criteria is set to use the link with the lowest AW, the link with the smallest AW is chosen.

	Cisco PNNI Network Planning Guide for MGX and SES Products, Release 5
Part Number OL-3847-01 Rev. D0, April, 2004		4-1