cord. DNS is not offered by all service provider SIP trunk offerings, but when it is, this is gener- ally a good method of load balancing. Even when it is not offered, this mechanism can Chapter 7: Design and Implementation Considerations 135 still be used to good effect on the enterprise side of the network border. This method is dependent on a predictable design of DNS server response time to ensure that post-dial delay (PDD) is minimal. The DNS SRV mechanism can also be used for load-balancing calls outbound from the CUBE to an attached softswitch. If DNS is used for this call path, the SIP INVITE retry timer might need to be tuned to constrain PDD for outbound calls, as shown in Example 7-10. Example 7-10 SIP Retry Timers sip-ua retry invite 2 CUCM Route Groups and Route Lists When connecting a CUCM to a cluster of border elements for PSTN SIP trunk access, its Route Group and Route Lists constructs can be used to implement a load balancing algo- rithm for presenting calls outbound from the enterprise to the PSTN. Other SIP softswitches and IP-PBXs most likely have similar alternative routing capabilities that can be used in a similar manner. The operation of this mechanism is shown in Figure 7-8. Route Group1 Route Route Pattern Route Cisco Unified CM Group2 Cisco Unified Border Element Cluster Figure 7-8 CUCM Route Groups and Route Lists Configure a Route Group on CUCM pointing to each individual border element. Aggregate these Route Groups into a Route List that points to the SIP trunk. Configure a Route Pattern in the CUCM dial-plan to route calls of the appropriate dialed number terns to this Route List. Configure CAC on the individual CUBEs to refuse calls under overload conditions, forcing CUCM to reroute to the next Route Group in the Route List. Cisco Unified SIP Proxy The Cisco Unified SIP Proxy can be used with a cluster of border elements as a logical large-scale SIP trunk network border interface to the attached softswitches. That is, the attached softswitches on both the service provider and ent