Notes on SPVCs

The original Cisco multiservice switching SPVC implementation follows the dual-endpoint SPVC model. In this model, an SPVC connection consists of two endpoints: an SPVC master endpoint and an SPVC slave endpoint.

The following list details their functions:

• The master endpoint owns and is responsible for routing and signaling. It controls the SPVC setup.

• The slave endpoint is a passive endpoint that is responsible for accepting or rejecting the SPVC setup.

This behavior is shown in Figure 10-16.

Figure 10-16. Dual-Endpoint SPVC Model

In a dual-endpoint SPVC model, both endpoints must be provisioned with the same service category and traffic parameters for the slave endpoint to accept the call. The slave endpoint is persistent.

This model is more robust than a single-endpoint SPVC model and has the following advantages:

• AIS generation on slave endpoint A persistent slave endpoint sends an alarm indication to the attached CPE if the SPVC is derouted.

• Statistics at the slave endpoint Statistics and counters can be kept because the slave endpoint is persistent.

• SVCs and SPVCs on the same interface Because the VPI/VCI pair is reserved at a persistent slave endpoint, an incoming SVC setup cannot use them.

• ATM-FR and ATM-CE service interworking If a slave endpoint is on a Frame Relay interface, Frame Relay traffic parameters are explicitly specified. Currently, no standard exists for signaling Frame Relay parameters across ATM, so a single-endpoint SPVC model must use proprietary signaling. The same holds true for ATM-to-circuit emulation interworking.

• Protection against configuration errors A dual-endpoint SPVC model protects users from making an SPVC call to the wrong ATM switch or port because the slave endpoint needs to be configured for the call to be accepted.

Given these advantages, a dual-endpoint SPVC model is normally preferred. In the case of interworking between a multiservice switching platform and a platform supporting only the single-endpoint SPVC model, there are different options. The multiservice switching platform can be configured with a master SPVC endpoint, and no configuration is required in the other platform. The multiservice switching platform can also be configured with a slave SPVC endpoint. However, the other platform needs to be configured, and all service categories and traffic parameters need to match between both endpoints. You can use the formula shown in Equation 10-1 to translate traffic parameter values in different units.

In particular, Table 10-6 summarizes the service category mappings to use when you configure SPVCs between a multiservice switching platform and a Cisco IOS Software-based platform.

Table 10-6. Service Category Mappings for SPVCs

Cisco IOS Software Platform Service Class	MSS Platform Service Class
cbr	cbr2(11)
vbr-rt	vbr1rt(2)
vbr-nrt	vbr1nrt(5)
ubr	ubr1(8)
abr	abrstd(10)

Current PNNI software in multiservice switching platforms also supports single-endpoint SPVC model and nonpersistent SPVC slave endpoints. In this case, a PNNI node can be configured so that an SPVC or SPVP request can override existing SVCs and SVPs using the requested VPI/VCI. This is because an SVC can take a different VPI/VCI pair but an SPVC endpoint cannot. The command cnfsvcoverride is provided for that functionality; it defaults to no override. On the other end of the spectrum, pnports can be configured to block incoming SPVC requests for nonpersistent endpoints. This is achieved with the parameter -nonpersblock in the command cnfpnportcc, which configures the PnPort call control. Single-endpoint SPVCs can be released with the command clrspvcnonpers.

Another new feature allows persistent slave endpoints in a dual-endpoint SPVC model to have a nodal traffic conformance tolerance factor for allowing SPVC request acceptance with different traffic parameters. This tolerance factor is configured as a percentage from the configured master endpoint's traffic parameters. The command cnftrftolerance lets you specify this nodal percentage, which defaults to 5 percent.