Cisco Systems Network Router Shortest Path Table Routing, Maximum Cell Rate, Shortest Path Tables

Chapter 4 Planning Intermediate Route Selection

How MGX and SES Nodes Select Routes

route more calls for the link. Of course, if link users suddenly start using all link resources, some user-compliant traffic may be discarded when congestion occurs. Bandwidth overbooking can be configured on a per-service-class-basis for each interface in the node.

Note Beginning with Release 3.0, Cisco MGX and SES nodes also support connection overbooking, which is configured with the addcon command. When per-service-class overbooking and connection based overbooking are both configured, both are applied simultaneously to each affected connection.

For more information, see the Cisco MGX 8850 (PXM1E/PXM45), Cisco MGX 8950, Cisco MGX 8830, and Cisco MGX 8880 Configuration Guide, Release 5 or see the appropriate service module configuration guide.

For CBR, rtVBR, and nrtVBR traffic, the advertised AvCR represents the bandwidth available for calls. For ABR traffic, AvCR is the capacity available for minimum cell rate (MCR) reservation. AvCR does not apply to UBR traffic.

The AvCR for a route is equal to the lowest link AvCR along the route.

Maximum Cell Rate

The maximum cell rate (maxCR) is a static value that is configured for each logical interface and can be configured separately for each service class. The maxCR represents the maximum throughput available for PNNI connections and cannot be modified by the overbooking factor. To block traffic for a particular service class over a link, set the maxCR for that service class to 0.

The maxCR for a route is equal to the lowest link maxCR along the route.

Shortest Path Table Routing

Most routing attempts begin with a search for a route in the shortest path tables. The following sections introduce the shortest path tables and explain how the tables are used by SVCs, SVPs, SPVCs, and SPVPs.

The Shortest Path Tables

The PNNI routing protocol automatically builds shortest path tables (SPTs) that list optimized routes for each destination address. When an MGX or SES node receives a call request, it compares the destination ATM address with the addresses and address prefixes in the node’s routing tables. The node looks for a match between the first 19 bytes of the destination address and the address prefixes in its database. The longest match determines the routes that are eligible. If there is just one route for the longest matching entry, and if that route meets the QoS requirements for the call, that is the route selected.

When multiple routes are available for the longest match, other route selection parameters are used to determine the optimum route.

Note Border nodes can be configured with a 0-length prefix, which matches all ATM addresses. This 0-length prefix serves as a default destination or route for all calls that do not match up to a longer ATM address or prefix within the PNNI network. When a border nodes uses AINI or IISP links to communicate with

	Cisco PNNI Network Planning Guide for MGX and SES Products, Release 5
Part Number OL-3847-01 Rev. D0, April, 2004		4-3