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