Fixed Interfaces

Early routers were little more than Unix workstations and PCs equipped with two Ethernet
interfaces. The first fixed-purpose routers were typically fixed interface as well—there was no
provision for adding an additional interface or a new type of interface. As router products
evolved, the capability to add modularity to the products increased. A fixed-interface router
cannot be expanded, so one with two Ethernet interfaces will always have only two Ethernet
interfaces. When you need a third, you must replace the router or augment it with another.
Fixed-interface routers typically reduce the costs associated with acquisition, which directly
relates to the initial capital expense. Many organizations try to reduce capital costs, even when
this leads to ultimate replacement requirements. In addition, fixed-interface routers are simpler
to install than modular routers, especially by less experienced staff and vendors. Fixed-interface
equipment lacks an upgrade path, however. It is impossible to add features without requiring
a complete replacement of the equipment. Replacing equipment can quickly offset the savings
you made with the initial purchase. Therefore, designers should seriously evaluate the life span
of the equipment and the growth potential for the environment before they make any irreversible
decisions. Typically, sites with more than 30 users will quickly outgrow fixed-configuration
routers, although different environments yield different thresholds.
Cisco offers two alternatives to the fixed router. The modular router enables network modules
or port adapters to be installed by supplying the type and volume of interfaces needed; this
is discussed in the next section. In addition, routers are also available for expansion with fixed
interfaces and one or more modular ports. The Cisco 1600 is a good example of this hybrid
router type and is discussed later in this chapter.