erprise sides are unaware of the individual elements, or the number of them, in the CUBE cluster. This is a handy mechanism when: 136 SIP Trunking You build large-scale SIP trunks where the number of border elements exceed the two IP addresses given by your provider. You want to grow the SIP trunk capacity over time without affecting the configura- tions of the attached softswitches on either side of the border. The Cisco Unified SIP Proxy is responsible for the load balancing over the individual bor- der elements, keeps track of their loads, and reroutes traffic when a particular element is overloaded or unavailable. The operation of this mechanism is shown in Figure 7-9. Cisco Unified SIP Proxy Service Provider Enterprise SIP Trunk SIP Trunk Service Provider SIP Cisco Unified Communications Cisco Unified Border Element Cluster Manager Figure 7-9 Cisco Unified SIP Proxy and Border Element Cluster In addition to load balancing, the Cisco Unified SIP Proxy offers many benefits to the SIP trunk interconnect: Hides the size of the border element pool from the attached softswitch configurations. Offers policy-based SIP trunk call routing such as time-of-day and least-cost routing. Offers powerful SIP Normalization capabilities. Offers graceful service degradation for upgrades or maintenance of the border ele- ments. Offers an easy way to expand the capacity of your SIP trunk when your needs grow. Offers intrinsic redundancy because there isn't a single border element but a cluster of them. (The SIP proxy itself must, of course, be deployed in a redundant configura- tion; otherwise, it becomes a single point of failure.) PSTN TDM Gateway Failover An easy and cost-effective way to provide redundancy and failover for a SIP trunk is sim- ply to reroute calls to your already existing TDM gateways when the SIP trunk is not available or overloaded. This method provides a ready migration path while you ramp up SIP trunk traffic to full production and enables you more time to design and implement Chapter 7: Design and Impleme