The last form of routing discipline is
hybridization. The balanced hybrid routing protocols use distance-vector metrics
but emphasize more accurate metrics than conventional distance-vector protocols.
They also converge more rapidly than distance-vector protocols but avoid the
overheads of link-state updates. Balanced hybrids are event driven rather than
periodic and thereby conserve bandwidth for real applications.
Although "open" balanced hybrid protocols exist, this form is
almost exclusively associated with the proprietary creation of a single company,
Cisco Systems, Inc. Its protocol, Enhanced Interior Gateway Routing Protocol (EIGRP),
was designed to combine the best aspects of distance-vector and link-state
routing protocols without incurring any of their performance limitations or
penalties.
Performance Characteristics of Hybridized Routing
One of the most difficult, yet critical, tasks that must be surmounted when
building an internetwork is the selection of a routing protocol. As the
preceding sections indicate, a rich palette of options await the prospective
internetwork architect. Although the preceding overviews should help you to
differentiate among the various classes of dynamic routing protocols, this is
just the beginning. You still need to select a specific routing protocol, or
protocols, from the variety that may be available in each class.
One of the best ways to start narrowing down the list of potential protocols
is by evaluating each protocol's performance characteristics relative to
projected requirements. Unlike hardware, you can't just compare routing
protocols' packets-per-second or bandwidth ratings. They don't exist! Instead,
you should look at how effectively each protocol performs the various tasks that
support internetworking.
Two of the most important of these tasks are convergence and route
calculation. The remaining sections of this chapter examine each of these
concepts in more detail and should adequately prepare you for the more detailed
examinations of individual protocols in the following chapters.
Note: Of course, many other performance attributes must be
considered, including maximum network diameter and how well a given protocol
accommodates heavy traffic loads. Such characteristics, however, tend to be more
applicable to specific protocols than to the three classes of dynamic routing
protocols identified earlier in this chapter.