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Destination 10000/8 10000/8 Bo 19216810/24 1721600/16 1722400/16 1921682000/24 1921682000/24 1722000/16 1722000/16 1723100/16 10000/8 Roscoe 1721600/16 1723100/16 1722000/16 10000/8 19216810/24 19216810/24 1921682000/24 1921682000/24 1722400/16 1722400/16 Luke 1723100/16 10000/8 0000/0
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Next Hop 1722011 1722411 1722412 1721612 1722412 1721612 1721612 1721612 1723112 1721611 1722012 1722012 1721611 1721611 1722012 1723111
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With this configuration, if Daisy forwarded a packet to Bo for the 10000 network, Bo would forward the packet to Roscoe 100 percent of the time Now you should understand what static routing is capable of The bottom line is, if you have a relatively small and simple network, static routing is probably your best choice If you have redundant links, you can still use static routing; but, in most cases, you would be better served by a dynamic routing protocol
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Dynamic routing, on the other hand, doesn't suffer from many of static routing's limitations The basic idea of dynamic routing is that a special protocol, called a routing protocol, communicates routes between the routers in the topology For instance, take a look at the network in Figure 22-9
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Figure 22-9: The initial network for the dynamic routing example The known networks for Figure 22-9 are as follows:
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Scully 1, 4, 3 Mulder 4, 5, 8, 10 Krychek 2, 3, 5, 6 Skinner 6, 7, 8, 9 Doggett 9, 10, 11
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This network has five routers in a partial (or hybrid) mesh For the sake of brevity, I have listed only their known networks (rather than the entire routing table) and have simplified the networks down to a single number At boot, each router would know about only the networks to which they are directly connected With static routing, for each router to "learn" the routes to all networks in this topology, you would need to manually enter all primary and backup routes manually This task would be a bit difficult, as well as be prone to the same packetpassing issues you saw earlier A dynamic routing protocol, on the other hand, automatically informs the other routers of all routes it knows about at regular intervals These "information" packets are called advertisements or routing updates, depending on the specific protocol in question These updates allow all routers to automatically learn about all routes (primary and backup) Note The following example has been greatly simplified to be non-protocol specific and to illustrate the basics of routing updates The purpose is not to show you how dynamic routing protocols work in real life (that is covered in s 23 26), but to give you a basic understanding of the routing update process To help you see how dynamic routing works, let's walk through the update process with the example network In Figure 22-10, all routers are sending out their initial advertisement to all other neighboring routers So Scully, for example, sends an advertisement to Mulder and Krychek containing information about her known networks (at this point, only networks 1, 3, and 4)
Figure 22-10: The initial advertisement in the example network At the time of the initial advertisement, the known networks for Figure 22-10 are as follows:
Scully 1, 4, 3 Mulder 4, 5, 8, 10 Krychek 2, 3, 5, 6 Skinner 6, 7, 8, 9 Doggett 9, 10, 11
After receiving the updates, the routers update their respective tables and, at the next update interval, send out a new update containing the updated list of networks, as shown in Figure 22-11 The dynamically learned entries in the routing table are marked in italic type, and the routers whose networks were learned from are listed in parentheses
Figure 22-11: The second update in the dynamic routing example At the time this second update is sent, the known networks for Figure 22-11 are as follows (with the router the network was learned from in parentheses):
Scully 1, 2 (Krychek), 3, 4, 5 (Mulder, Krychek), 6 (Krychek), 8 (Mulder), and 10 (Mulder) Mulder 1 (Scully), 2 (Krychek), 3 (Scully, Krychek), 4, 5, 6 (Krychek, Skinner), 7 (Skinner), 8, 9 (Skinner, Doggett), 10, and 11 (Doggett) Krychek 1 (Scully), 2, 3, 4 (Scully, Mulder), 5, 6, 7 (Skinner), 8 (Mulder, Skinner), 9 (Skinner), and 10 (Mulder)
Skinner 2 (Krychek), 3 (Krychek), 4 (Mulder), 5 (Krychek, Mulder), 6, 7, 8, 9, 10 (Mulder, Doggett), and 11 (Doggett) Doggett 4 (Mulder), 5 (Mulder), 6 (Skinner), 7 (Skinner), 8 (Mulder, Skinner), 9, 10, 11
After looking at Figure 22-11, you are probably thinking that this thing just got very complicated really quick! Unfortunately, that complexity is one of the biggest drawbacks of dynamic routing (and one of the reasons I believe that the more you understand about networking, the more surprised you will be that it actually works at all!) Eventually, however, the network will settle down and every router will know of every path to every network This state is known as convergence, and it is shown in Figure 22-12
Figure 22-12: The fully converged network At full convergence, the known networks for Figure 22-12 are as follows (with the router the network was learned from in parentheses):
Scully 1, 2 (Krychek, Mulder), 3, 4, 5 (Krychek, Mulder), 6 (Krychek, Mulder), 8 (Krychek, Mulder), 9 (Krychek, Mulder), 10 (Krychek, Mulder), and 11 (Krychek, Mulder) Mulder 1 (Scully, Krychek, Skinner, Doggett ), 2 (Scully, Krychek, Skinner, Doggett), 3 (Scully, Krychek, Skinner, Doggett), 4, 5, 6 (Scully, Krychek, Skinner, Doggett), 7 (Scully, Krychek, Skinner, Doggett), 8, 9 (Scully, Krychek, Skinner, Doggett), 10, and 11 (Scully, Krychek, Skinner, Doggett) Krychek 1 (Scully, Mulder, Skinner), 2, 3, 4 (Scully, Mulder, Skinner), 5, 6, 7 (Scully, Mulder, Skinner), 8 (Scully, Mulder, Skinner), 9 (Scully, Mulder, Skinner), 10 (Scully, Mulder, Skinner), and 11 (Scully, Mulder, Skinner) Skinner 1 (Krychek, Mulder, Doggett), 2 (Krychek, Mulder, Doggett), 3 (Krychek, Mulder, Doggett), 4 (Krychek, Mulder, Doggett), 5 (Krychek, Mulder, Doggett), 6, 7, 8, 9, 10 (Krychek, Mulder, Doggett), and 11 (Krychek, Mulder, Doggett) Doggett 1 (Mulder, Skinner), 2 (Mulder, Skinner), 3 (Mulder, Skinner), 4 (Mulder, Skinner), 5 (Mulder, Skinner), 6 (Mulder, Skinner), 7 (Mulder, Skinner), 8 (Mulder, Skinner), 9, 10, and 11
Note If you have a sharp eye, you may also have noticed one of the other major drawbacks of dynamic routing: routing loops Preventing routing loops is a major chore for most dynamic routing protocols, and all of them handle it a bit differently You will learn about the various solutions to routing loop problems in later chapters
Now that you've seen the basics of what a routing protocol does, let's take a look at the goals of all routing protocols:
To reduce administrative effort by dynamically filling the routing tables with routes to all networks When more than one route to a given network is available, either o To place the best route in the table, or o To place multiple routes in the table and load balance across the routes To automatically remove invalid routes from the table when a failure (direct or indirect) occurs If a better route is heard, to add that route to the table To eliminate routing loops as quickly as possible
These goals are the same for all routing protocols, regardless of the routing algorithm used As you will see in upcoming chapters, sometimes meeting all of these goals is easier said than done Routing protocols are typically classified by the type of logic they use The three primary algorithms used by routing protocols are listed here, along with a brief explanation of each:
Distance vector This algorithm is the most common type of routing logic used today Distance vector routing is sometimes called "routing by rumor" Basically, a distance vector protocol yells out to all directly connected neighbors, "Hey guys, I know about all of these networks!" The neighbors go, "Great, let me add that to my table And, by the way, I know about all of these networks!" and so on None of the routers actually know about these networks (unless, of course, they are directly connected), they just think they know about all of the networks If one router has bad information, it passes this invalid information on to all other routers without realizing that it is wrong For this reason, distance vector protocols require complex algorithms to "doubt" any updates they hear to prevent large-scale routing loops Distance vector protocols covered in this book include Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) I also consider Enhanced IGRP (EIGRP) to be a distance vector protocol (albeit, one on steroids), but Cisco classifies it as a "balanced hybrid" (probably because distance vector has such a bad rap) Link state Link state routing logic operates on the Dykstra Shortest Path First (SPF) algorithm, and it operates a bit differently than distance vector Basically, link state protocols build a "map" of the network, so they inherently have a better idea of where everything is located than distance vector protocols do This advantage makes link state protocols much more graceful and sophisticated, but it also makes them harder to understand and implement properly Very few routing protocols use the link state logic, and the only one this book covers is Open Shortest Path First (OSPF) Balanced hybrid This algorithm is Cisco's term for EIGRP's routing logic Personally, I think Cisco just made this designation up for marketing purposes; but because they did classify it this way, I will cover it in this manner The idea behind the balanced hybrid designation is that it includes features of both distance vector and link state algorithms For instance, the core logic behind EIGRP is basically distance vector, but EIGRP transmits additional topology information in order to "map" the network, like link state The only balanced hybrid protocol discussed in this book is EIGRP
Here's a quick analogy to help describe the difference in these routing logic types Pretend your routing protocol is a traveler lost in the desert, wandering in circles looking for water, and it comes across a fork in the path If it were a distance vector protocol, it would read the sign that says "water over here" and follow that path, not realizing that some kids were having fun and switched the sign If it were link state, it would take the time to look at its meticulously drawn map, and (eventually) figure out the correct path If it were balanced hybrid, it would ask for directions In addition to being classified by logic type, routing protocols are also classified by their intended use: either as an interior gateway protocol (IGP) or an exterior gateway protocol (EGP) IGPs are typically limited in the size of networks they can adequately support (although, with a little tweaking, some of them can support rather large networks); and they are, therefore, usually a bit simpler than EGPs IGPs are designed to route within autonomous systems (ASs) An AS is a fancy way of saying a network controlled by a single administrative entity LANs are a single AS, and so are most corporate WANs The Internet, however, is a collection of several ASs The AS concept is examined in more detail in later chapters EGPs are built to support huge networks An EGP's primary purpose is to route between ASs Due to the complexities involved with EGPs, they will not be covered in this book Now let's take a look at a technique to deal with the common problem of having a router make routing decisions based on information from multiple dynamic routing protocols The technique used by Cisco to combat this issue is called administrative distance, and it is described in the next section
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