"Cost" Versus "Availability"

Internetwork costs fall into two general categories :
- One-time fixed costs ( Equipments purchased, such as HW & SW )
- Recurring costs (  Monthly circuit fees from ISP, Ongoing Salaries of Engineers )

When used at its maximum, each becomes a trade-off of the other.  As a result, the Design Professional must carefully weigh the relative importance of resource availability and overall cost.  If you could make a network completely fault-tolerant, this could be done only at a cost that most businesses would consider unacceptable.  Beware of designing redundancy just for the sake of redundancy.