 are relatively stable anyway (IBM's Systems Network Architecture, SNA, for example). More common are dynamic routing protocols, in which network devices make their own decisions about optimum route selection. They do this in the following general way: They pay attention to the network around them; collect information from their neighbors about best routes to par- ticular destinations based on such parameters as least number of hops, least delay, lowest cost, or highest bandwidth; archive those bits of infor- mation in tables; and then selectively flush the tables periodically to ensure that the information contained in them is always as current as possible. Because dynamic routing protocols assume intelligence in the switch and can therefore reduce the amount of human intervention required, they are commonly used; in fact, they are the most widely deployed routing protocols. Dynamic routing protocols are further divided into two subcategories, centralized and distributed. Centralized routing protocols concentrate the route decision-making processes in a single node, thus ensuring that all nodes in the network receive the same and most current information possible. When a switch or router needs routing information that is not contained in its own table, it sends a request to the root node asking for direction. There are significant downsides to this technique. First, by con- centrating the decision-making capability in a single node, the likelihood of a catastrophic failure is dramatically increased. If that node fails, the Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols entire network's ability to seek optimal routing decisions fails. Second, because all nodes in the network must go to that central device for rout- ing instructions, a significant choke point can result. There are several options that can reduce the vulne