I recently asked Brownlee Thomas, principal analyst for Global Telecom Services at Forrester Research, what advice she gives to corporate network managers when buying IP services. Here are her suggestions:1. In writing an RFP for IP services, you need to figure out which ISPs offer the best fit for you. Provide bidders with as many details as possible about your network - where it goes, what cities it covers, what type of applications it carries, etc. Then figure out which ISP networks are the closest match to yours. If you need a Tier 1 provider, look for a service provider that owns or controls the network infrastructure that's closest to your requirements. A Tier 1 provider also should provide market-leading service-level agreements (SLA).2. Ask the ISPs bidding on your RFP to provide you with historical performance data of between five or 10 important city pairs on your network. For example, ask them to show you latency statistics between New York and Los Angeles or New York and Brussels. You'll want to analyze between three and six months worth of data. Compare this data to the SLAs that the ISPs offer to see which service providers are most likely to meet their performance guarantees.3. Other factors to consider in choosing an ISP are financial viability and organizational stability. In this economy, most ISPs have financial problems. Many ISPs have gone through bankruptcy proceedings or changed ownership in recent years. Look for an ISP that's a consistent performer and is showing flat revenues if not growth.4. Consider price last. Price should never account for more than 10% or 15% of the weighting of your purchasing factors on a 100% scale. Most important is the overall network match between the buyer and the ISP, which should account for 30% to 35% of the weighting. Network performance and customer service are other factors to consider.