Resource-Based CAC 589

fail whenever the single terminating gateway is too busy. In addition, in enterprise networks the probability of congestion is typically higher in the IP cloud than in the number of terminating POTS trunks. In the SP networks discussed earlier, congestion is more common in the terminating POTS trunks than in the IP cloud.

In spite of these limitations, RAI can still be used for enterprise networks provided the gateway to PBX connections at the remote sites consist of T1/E1 trunks. If a terminating gateway is too busy, it triggers a PSTN reroute instead of selecting an alternate gateway as in the SP network situation.

RAI Operation

The discussion of where and how RAI is used in SP and enterprise networks clearly shows that RAI is most useful in situations where multiple terminating gateways can reach the same destination, or called, phone number. However, RAI has value in any situation where there is a desire to prevent a call from being routed to a gateway that does not have the available POTS capacity to terminate the call.

When a gatekeeper receives an RAI unavailable indication from a gateway, it removes that gateway from its gateway selection algorithm for the phone numbers that gateway would normally terminate. An RAI available indication received later returns the gateway to the selection algorithm of the gatekeeper.

RAI is an optional H.323 feature. When you implement a network, therefore, it is prudent to verify that both the gateways and gatekeepers support this feature. Cisco gatekeepers support RAI. Cisco gateway support for RAI is detailed in a later section of this chapter.

RAI Configuration

RAI on the gateway is configured with high-water- and low-water-mark thresholds, as shown in Figure 8-19. When resource usage rises above the high-water mark, an RAI unavailable indication is sent to the gatekeeper. When resource availability falls below the low-water mark, an RAI available indication is sent to the gatekeeper. To prevent hysteresis based on the arrival or disconnection of a single call, the highand low-water marks should be configured 10 percentage points apart. (Hysteresis refers to the process of switching a feature on, then off, then on, and so on, over and over again, as would happen if the highand low-water marks were configured to very similar values.)

To configure RAI, use the following gateway configuration command:

resource threshold [all] [high %-value] [low %-value]

To configure an RAI unavailable resource message to be sent to the gatekeeper at a high-water mark of 90 percent and a RAI available resource message to be sent to the gatekeeper at a lowwater mark of 80 percent, enter the following command:

resource threshold high 90 low 80