Here is the link to the full story of "Debunking the Myth of a Single Vendor Network"
To give a quick summary here are some points from the Gartner report.
Introducing a second vendor into the network infrastructure will have no long-term impact on operational costs for organizations following best practices.
Introducing a second networking vendor will reduce total cost of ownership (TCO) for most organizations by at least 15% to 25% over a five-year time frame.
We did not encounter one example were operational cost savings would offset the equipment cost premium that Cisco generally charges.
Most organizations that introduced a second vendor report a lasting decrease in network complexity, compared with an all-Cisco network.
Network operations teams that are already using some form of multivendor management tools for fault alerting, configuration management or performance management are well-positioned to take advantage of the second-vendor opportunity.
Network architects and CIOs must consider alternative network vendors to ensure that they deliver a functional network solution at an appropriate cost point.
Network operations teams should invest in multivendor-capable tools to help enable the organization to deal with a second vendor in their infrastructure, and to improve the operational capabilities with their incumbent solution.