 support stateful failover if the higher layers of software support application-level checkpointing and the basic router keepalive. The operation of this mechanism is shown in Figure 7-6. Chapter 7: Design and Implementation Considerations 133 Cisco Unified Communications Primary interface FastEthernet0/0 Manager 10.1.1.2 ip address 10.1.1.2 255.255.255.0 standby ip 10.1.1.1 standby priority 200 standby preempt HSRP 10.1.1.1 interface FastEthernet0/0 ip address .1.3 255.255.255.0 standby ip 10.1.1.1 Secondary standby priority 100 10.1.1.3 Cisco Unified Border Elements Figure 7-6 Using HRSP for Redundancy Enterprise TDM gateways do not offer stateful failover redundancy because the session capacity per gateway is limited; therefore, the impact of a failure is limited. If an individ- ual CUBE carries no more sessions than the average enterprise TDM gateway, there might not be a reason to expend the cost on deploying high-end hardware with stateful failover capability on the border element either. Instead, border element clustering can provide effective redundancy, as it does for TDM gateways. Clustering (N+1) Redundancy via clustering, or N+1 redundancy, means there are duplicate platforms inde- pendent of each other and each carries a fraction of the traffic, together providing a high session count SIP trunk. There is no state sharing or keepalives between the components, and if a single element is lost, some calls drop, but it is not the entire SIP trunk that goes down. The CUBE can be deployed in a clustering architecture with load balancing over the indi- vidual components managed by the attached devices or by a SIP proxy element. (Load balancing methods are explored further in the next section.) A clustering architecture has the advantage of a pool of smaller elements, each of which can be taken out of service and upgraded without affecting the entire SIP trunk. The cluster can also be spread out over several buildings or geographic locations to enhance redundancy concerns about the impact of a power loss or a