 this highway, but it's rush hour and you'll be stuck for days if you go that way. I'd take the surface street. It's a little less comfortable, but there's no con- gestion." By stopping at a series of gas stations as you traverse the coun- try and asking for help, you will eventually reach your destination, but the route may not be the most efficient. That's okay, though, because you will never have to worry about getting stuck with bad directions. Clearly the first example of these is connection oriented; the second is connec- tionless. Connection-oriented travel is a far more comfortable, secure way to go; connectionless is riskier, less sure, more flexible, and much more fun. Obviously, distributed routing protocols are centrally important to the traveler as well as to the gas station attendant who must give them reliable directions, and equally important to routers and switches in con- nectionless data networks. Distributed routing protocols fall into two categories: distance vector and link state. Distance vector protocols rely on a technique called table swapping to exchange information about network topology with each other. This information includes destination/cost pairs that allow each device to select the least cost route from one place to another. On a sched- uled basis, routers transmit their entire routing tables on all ports to all adjacent devices. Each device then adds any newly arrived information to its own tables, thus ensuring currency. The only problem with this technique is that it results in a tremendous amount of management traf- fic (the tables) being sent between network devices, and if the network is 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 relatively static-that is, changes in topology don't happen all that often -then much of the information is unnecessary and can cause serious congest