What, if anything, makes a multi-vendor (wide-area) network successful and worth the risks over the "safe" single-vendor network nobody gets fired for buying (you can probably guess what vendor Powers my network now).
I like thinking of where an organization wants to be on a risk/reward spectrum. Newer and/or point solution vendors exist to leapfrog the status quo and give you an advantage, which they do very well. This is particularly true on the optical side, where lasers, components, and software have all changed dramatically. The price you pay is risk. If you don't have capacity problems or take an incumbent role, then you would prefer a defensive, conservative strategy favoring the single-vendor solution. ------ We actually consider risk and choice of vendor to be to be separate principle areas. It is certainly easy to be risky with your use of technology while embracing a single vendor approach, think VoIP. I'd consider ripping out PBX's in favor of VoIP to be risky, and it is certainly a leading edge approach, yet you can buy this type of solution today from the larger vendors. In one example we worked with a customer who was considering using L3 switches with newly released WAN blades in the core of their network. The conservative approach was to stick with a more proven platform, the risky approach was to go with the newer product. Their principles were to use a single vendor, but they also had a principle to use only stable and mature technologies in the core, so they went with the more traditional routing platform. BTW, we also separate out vendor risk - e.g. we create a principle that determines how much vendor risk to accept. The extremes are only buying from proven vendors vs. buying from start-ups with low market share, provided there is some competitive advantage to be gained. Irwin