ngineering departments) can use a 10:1 ratio. Contact center deployments should use a 1:1 ratio, and phones in this context include both live agents and automated ports serving Interactive Voice Response (IVR) front-end applications. SIP Trunk Monitoring Several generic IP mechanisms can monitor the health of a network element, such as an Internet Control Message Protocol (ICMP) Ping. Although these are useful, they provide only Layer 3 health. The SIP protocol specifies an Out-of-Dialog (OOD) Options Ping method in RFC-3261 that provides a Layer 7 health indication of a SIP endpoint. The OOD Options Ping method can provide a health check for a SIP trunk and enables attached devices to reroute traffic upon a failure of any one element in the path. Note that it is a per-hop method and that several Pings might need to be configured to provide end-to-end failure detection on a SIP trunk. This method is illustrated in Figure 7-11. Cisco Unified Service Cisco Unified Cisco Unified Border Communications Provider SIP SIP Proxy Element Cluster Manager (1) Options Ping (3) Options Ping (2) Options Ping (5) Options Ping (4) Options Ping Figure 7-11 SIP Trunk Monitoring Using Options Ping If the Options Ping between the elements fails (in the direction indicated in Figure 7-11), the following actions are taken: Step 1. The service provider fails over to the secondary IP address for the SIP trunk, if available, or reroutes calls destined to the enterprise. Step 2. The Cisco Unified SIP Proxy marks a border element as down and reroutes calls to alternative border elements in the cluster until it comes back up. Chapter 7: Design and Implementation Considerations 139 Step 3. The Cisco Unified SIP Proxy marks CUCM as down and rejects incoming calls from the service provider. Step 4. When supported (a future capability), this path allows a CUCM to mark the SIP trunk as down and use its alternative routing logic to place outgoing calls. Step 5. The Cisco Unified SIP marks the SIP trunk to the service provider as down and rejects i