Thus spake "vijay gill" <vgill@vijaygill.com>
Unfortunately, while this sounds excellent in theory, what really happens is that you have a large chunk of equipment in the network belonging to vendor X, and then you introduce vendor Y. Most people I know don't suddenly throw out vendor X ... . People don't do that because it costs a lot of capital and opex. So now we have a partial X and partial Y network, X goes down, and chances are your network got hammered like an icecube in a blender set to Frappe.
I think an important factor in this is that multiple vendors are rarely deployed within redundant pairs, which at least has a hope of surviving one vendor's cascading software faults. More often, each vendor's products are used universally in particular tier(s) in the network, such that a failure of one vendor may leave you with no access but a working core, or vice versa. S Stephen Sprunk "Stupid people surround themselves with smart CCIE #3723 people. Smart people surround themselves with K5SSS smart people who disagree with them." --Aaron Sorkin