IP trunk is not from your data provider, there might be more incentive to do cen- tralized trunking. For example, if the offering comes with only two IP addresses, you can at most connect two sites to this offering. In this case, the two IP addresses are almost invariably used for redundancy purposes (as opposed to it representing two separate log- ical SIP trunk offerings), and both IP terminations could come into the same site of your network or could go to two different sites. It often makes sense to connect these to two different data centers or campus sites geographically separate from each other for maxi- mum disaster recovery benefits. Some SPs use these two IP addresses in a strict primary/secondary arrangement; others offer load balancing over both. Tip Discuss IP addressing of the SIP trunk offering with your provider because this scheme varies among the offerings, has design implications on your network, and can affect redundancy and failover design options. 96 SIP Trunking Understanding the Centralized Model with Direct Media Model Another variation to centralized and distributed SIP trunking worthy of discussion is a model where the signaling follows a centralized design, but the media flows directly from the remote sites to the SP network. This is often referred to as the direct media model and is illustrated in Figure 6-5. Softswitch Media Gateway SP VolP Session Border Campus Controller Border Element Signaling Integrated Service Router Branch with Border Offices Element Figure 6-5 Aggregated Signaling, Direct Media Model This model provides many of the benefits of the centralized model in the sense that dial- plan management, call routing decisions, and SIP trunk management (such as CAC) is done and viewed centrally. Yet it offers the media path and bandwidth optimization of the distributed model. Deploying this model in your network requires the following additional considerations: A Border Element is required in each site because the central signaling Border Element cannot provide media stream 