cy mechanisms dis- cussed next. Local and Geographical SIP Trunk Redundancy For redundancy purposes there are almost always multiple SIP trunk entry points into an enterprise network even in a largely centralized design. This ensures that calls have alter- native routing points if an equipment or building power failure occurs or a natural disas- ter in a particular region occurs. The only realistic alternative to multiple SIP trunk entry points is to have a single SIP trunk and maintain TDM gateway access to the PSTN for failover, a scenario discussed later in this section. For small, single-site businesses, cellular phone access might be a realistic alternative to a single SIP trunk, but this is rarely practi- cal for a multisite enterprise of any size. Consider three different areas of SIP trunk redundancy: Local redundancy: Most SIP trunk services offer at least two IP addresses. For local redundancy the physical medium is most likely shared and terminates into the same building on your premises. Local redundancy protects against equip- ment failure or power failure to a single piece of equipment. These two IP ad- dresses should ideally terminate onto two redundant border elements. Most providers offer either a primary/secondary or a load-balancing scheme that the enterprise can choose from. Geographic redundancy: Most medium-to-large enterprises prefer to bring in the two IP addresses or perhaps two different SIP trunks (that is, four IP addresses, each SIP trunk with local redundancy) into two separate buildings, likely data centers, in two different geographies. This protects against natural disasters and buildingwide power or other outages. 132 SIP Trunking Service provider redundancy: Some enterprises and contact centers get SIP trunks from two different providers, both for least-cost routing opportunities and for redun- dancy purposes. If one provider is having problems, the other provider's facilities can carry all traffic. This scheme is easy to implement for outbound traffic but harder (due to DID map