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• Routing Basics
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  What is Routing?
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  Routing is the act of moving information across an internetwork from a source to a destination.
  Along the way, at least one intermediate node typically is encountered. Routing is often contrasted
  with bridging, which might seem to accomplish precisely the same thing to the casual observer. The
  primary difference between the two is that bridging occurs at Layer 2 (the link layer) of the OSI
  reference model, whereas routing occurs at Layer 3 (the network layer). This distinction provides
  routing and bridging with different information to use in the process of moving information from
  source to destination, so the two functions accomplish their tasks in different ways.
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  The topic of routing has been covered in computer science literature for more than two decades, but
  routing achieved commercial popularity as late as the mid-1980s. The primary reason for this time
  lag is that networks in the 1970s were fairly simple, homogeneous environments. Only relatively
  recently has large-scale internetworking become popular.
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  Routing Components
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  Routing involves two basic activities: determining optimal routing paths and transporting
  information groups (typically called packets) through an internetwork. In the context of the routing
  process, the latter of these is referred to as switching. Although switching is relatively
  straightforward, path determination can be very complex.
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  Path Determination
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  A metric is a standard of measurement, such as path length, that is used by routing algorithms to
  determine the optimal path to a destination. To aid the process of path determination, routing
  algorithms initialize and maintain routing tables, which contain route information. Route
  information varies depending on the routing algorithm used.
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• Routing algorithms fill routing tables with a variety of information. Destination/next hop
  associations tell a router that a particular destination can be gained optimally by sending the packet
  to a particular router representing the ``next hop'' on the way to the final destination. When a router
  receives an incoming packet, it checks the destination address and attempts to associate this address
  with a next hop. Figure depicts a sample destination/next hop routing table.
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                   Destination/next hop associations determine the data's optimal path

• Routing tables also can contain other information, such as data about the desirability of a path.
  Routers compare metrics to determine optimal routes, and these metrics differ depending on the
  design of the routing algorithm used. A variety of common metrics will be introduced and described
  later in this chapter.
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  Routers communicate with one another and maintain their routing tables through the transmission
  of a variety of messages. The routing update message is one such message that generally consists of
  all or a portion of a routing table. By analyzing routing updates from all other routers, a router can
  build a detailed picture of network topology. A link-state advertisement, another example of a
  message sent between routers, informs other routers of the state of the sender's links. Link
  information also can be used to build a complete picture of topology to enable routers to determine
  optimal routes to network destinations.
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• Switching
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  Switching algorithms are relatively simple and are basically the same for most routing protocols. In
  most cases, a host determines that it must send a packet to another host. Having acquired a router's
  address by some means, the source host sends a packet addressed specifically to a router's physical
  (Media Access Control [MAC]-layer) address, this time with the protocol (network layer) address
  of the destination host.
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  As it examines the packet's destination protocol address, the router determines that it either knows
  or does not know how to forward the packet to the next hop. If the router does not know how to
  forward the packet, it typically drops the packet.
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• If the router knows how to forward the packet, it changes the destination physical address to that of the
• next hop and transmits the packet. The next hop may, in fact, be the ultimate destination host.
• If not, the next hop is usually another router, which executes the same switching decision process.
• As the packet moves through the internetwork, its physical address changes, but its protocol address
• remains constant, as illustrated in Figure .
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• The preceding discussion describes switching between a source and a destination end system. The
  International Organization for Standardization (ISO) has developed a hierarchical terminology that
  is useful in describing this process. Using this terminology, network devices without the capability
  to forward packets between subnetworks are called end systems (ESs), whereas network devices
  with these capabilities are called intermediate systems (ISs).
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• ISs are further divided into those that can communicate within routing domains (intradomain ISs) and
• those that communicate both within and between routing domains (interdomain ISs). A routing domain
• generally is considered to be a portion of an internetwork under common administrative authority that
• is regulated by a particular set of administrative guidelines.
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• Routing domains are also called autonomous systems. With certain protocols, routing domains can be
• divided into routing areas, but intradomain routing protocols are still used for switching both within and between
• areas.
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                         Numerous routers may come into play during the switching process

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• Routing Algorithms
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  Routing algorithms can be differentiated based on several key characteristics. First, the particular
  goals of the algorithm designer affect the operation of the resulting routing protocol. Second, various
  types of routing algorithms exist, and each algorithm has a different impact on network and router
  resources. Finally, routing algorithms use a variety of metrics that affect calculation of optimal
  routes. The following sections analyze these routing algorithm attributes.
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  Design Goals
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  Routing algorithms often have one or more of the following design goals:
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  . Optimality
  . Simplicity and low overhead
  . Robustness and stability
  . Rapid convergence
  . Flexibility
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  Optimality refers to the capability of the routing algorithm to select the best route, which depends
  on the metrics and metric weightings used to make the calculation. One routing algorithm, for
  example, may use a number of hops and delays, but may weight delay more heavily in the
  calculation. Naturally, routing protocols must define their metric calculation algorithms strictly.
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  Routing algorithms also are designed to be as simple as possible. In other words, the routing
  algorithm must offer its functionality efficiently, with a minimum of software and utilization
  overhead. Efficiency is particularly important when the software implementing the routing algorithm
  must run on a computer with limited physical resources.
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  Routing algorithms must be robust, which means that they should perform correctly in the face of
  unusual or unforeseen circumstances, such as hardware failures, high load conditions, and incorrect
  implementations. Because routers are located at network junction points, they can cause
  considerable problems when they fail. The best routing algorithms are often those that have
  withstood the test of time and have proven stable under a variety of network conditions.
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  In addition, routing algorithms must converge rapidly. Convergence is the process of agreement, by
  all routers, on optimal routes. When a network event causes routes either to go down or become
  available, routers distribute routing update messages that permeate networks, stimulating
  recalculation of optimal routes and eventually causing all routers to agree on these routes. Routing
  algorithms that converge slowly can cause routing loops or network outages.
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  In the routing loop displayed in Figure 5-3, a packet arrives at Router 1 at time t1. Router 1 already
  has been updated and thus knows that the optimal route to the destination calls for Router 2 to be the
  next stop. Router 1 therefore forwards the packet to Router 2, but because this router has not yet been
  updated, it believes that the optimal next hop is Router 1. Router 2 therefore forwards the packet
  back to Router 1, and the packet continues to bounce back and forth between the two routers until
  Router 2 receives its routing update or until the packet has been switched the maximum number of
  times allowed.
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                                           Slow convergence and routing loops can hinder progress
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•                                      
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• Routing algorithms should also be flexible, which means that they should quickly and accurately
  adapt to a variety of network circumstances. Assume, for example, that a network segment has gone
  down. As they become aware of the problem, many routing algorithms will quickly select the
  next-best path for all routes normally using that segment. Routing algorithms can be programmed to
  adapt to changes in network bandwidth, router queue size, and network delay, among other variables.
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• Algorithm Types
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  Routing algorithms can be classified by type. Key differentiators include:
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  . Static versus dynamic
  . Single-path versus multi-path
  . Flat versus hierarchical
  . Host-intelligent versus router-intelligent
  . Intradomain versus interdomain
  . Link state versus distance vector
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  Static Versus Dynamic
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  Static routing algorithms are hardly algorithms at all, but are table mappings established by the
  network administrator prior to the beginning of routing. These mappings do not change unless the
  network administrator alters them. Algorithms that use static routes are simple to design and work
  well in environments where network traffic is relatively predictable and where network design is
  relatively simple.
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  Because static routing systems cannot react to network changes, they generally are considered
  unsuitable for today's large, changing networks. Most of the dominant routing algorithms in the
  1990s are dynamic routing algorithms, which adjust to changing network circumstances by
  analyzing incoming routing update messages.
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• If the message indicates that a network change has occurred, the routing software recalculates routes
• and sends out new routing update messages. These messages permeate the network, stimulating routers
• to rerun their algorithms and change their routing tables accordingly.
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  Dynamic routing algorithms can be supplemented with static routes where appropriate. A router of
  last resort (a router to which all unroutable packets are sent), for example, can be designated to act
  as a repository for all unroutable packets, ensuring that all messages are at least handled in some way.
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• Single-Path Versus Multipath
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  Some sophisticated routing protocols support multiple paths to the same destination. Unlike
  single-path algorithms, these multipath algorithms permit traffic multiplexing over multiple lines.
  The advantages of multipath algorithms are obvious: They can provide substantially better
  throughput and reliability.
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  Flat Versus Hierarchical
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  Some routing algorithms operate in a flat space, while others use routing hierarchies. In a flat routing
  system, the routers are peers of all others. In a hierarchical routing system, some routers form what
  amounts to a routing backbone.
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• Packets from non-backbone routers travel to the backbone routers,where they are sent through the backbone
• until they reach the general area of the destination. At this point, they travel from the last backbone router
• through one or more non-backbone routers to the final destination.
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  Routing systems often designate logical groups of nodes, called domains, autonomous systems, or
  areas. In hierarchical systems, some routers in a domain can communicate with routers in other
  domains, while others can communicate only with routers within their domain.
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• In very large networks, additional hierarchical levels may exist, with routers at the highest hierarchical level
  forming the routing backbone.
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  The primary advantage of hierarchical routing is that it mimics the organization of most companies
  and therefore supports their traffic patterns well. Most network communication occurs within small
  company groups (domains).
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•  Because intradomain routers need to know only about other routers within their domain, their routing algorithms
• can be simplified, and, depending on the routing algorithm being used, routing update traffic can be reduced accordingly.
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  Host-Intelligent Versus Router-Intelligent
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  Some routing algorithms assume that the source end-node will determine the entire route. This is
  usually referred to as source routing. In source-routing systems, routers merely act as
  store-and-forward devices, mindlessly sending the packet to the next stop.
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  Other algorithms assume that hosts know nothing about routes. In these algorithms, routers
  determine the path through the internetwork based on their own calculations. In the first system, the
  hosts have the routing intelligence. In the latter system, routers have the routing intelligence.
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  The trade-off between host-intelligent and router-intelligent routing is one of path optimality versus
  traffic overhead. Host-intelligent systems choose the better routes more often, because they typically
  discover all possible routes to the destination before the packet is actually sent. They then choose the
  best path based on that particular system's definition of ``optimal.'' The act of determining all routes,
  however, often requires substantial discovery traffic and a significant amount of time.
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  Intradomain Versus Interdomain
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  Some routing algorithms work only within domains; others work within and between domains. The
  nature of these two algorithm types is different. It stands to reason, therefore, that an optimal
  intradomain- routing algorithm would not necessarily be an optimal interdomain- routing algorithm.
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  Link State Versus Distance Vector
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  Link- state algorithms (also known as shortest path first algorithms) flood routing information to all
  nodes in the internetwork. Each router, however, sends only the portion of the routing table that
  describes the state of its own links. Distance- vector algorithms (also known as Bellman-Ford
  algorithms) call for each router to send all or some portion of its routing table, but only to its
  neighbors. In essence, link- state algorithms send small updates everywhere, while distance- vector
  algorithms send larger updates only to neighboring routers.
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  Because they converge more quickly, link- state algorithms are somewhat less prone to routing loops
  than distance- vector algorithms. On the other hand, link- state algorithms require more CPU power
  and memory than distance vector algorithms. Link-state algorithms, therefore, can be more
  expensive to implement and support. Despite their differences, both algorithm types perform well in
  most circumstances.
  
  Routing Metrics
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  Routing tables contain information used by switching software to select the best route. But how,
  specifically, are routing tables built? What is the specific nature of the information they contain?
  How do routing algorithms determine that one route is preferable to others?
  Routing algorithms have used many different metrics to determine the best route. Sophisticated
  routing algorithms can base route selection on multiple metrics, combining them in a single (hybrid)
  metric. All the following metrics have been used:
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  Path Length
  Reliability
  Delay
  Bandwidth
  Load
  Communication Cost
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  Path length is the most common routing metric. Some routing protocols allow network
  administrators to assign arbitrary costs to each network link. In this case, path length is the sum of
  the costs associated with each link traversed. Other routing protocols define hop count, a metric that
  specifies the number of passes through internetworking products, such as routers, that a packet must
  take en route from a source to a destination.
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  Reliability, in the context of routing algorithms, refers to the dependability (usually described in
  terms of the bit-error rate) of each network link. Some network links might go down more often than
  others. After a network fails, certain network links might be repaired more easily or more quickly
  than other links. Any reliability factors can be taken into account in the assignment of the reliability
  ratings, which are arbitrary numeric values usually assigned to network links by network
  administrators.
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  Routing delay refers to the length of time required to move a packet from source to destination
  through the internetwork. Delay depends on many factors, including the bandwidth of intermediate
  network links, the port queues at each router along the way, network congestion on all intermediate
  network links, and the physical distance to be travelled. Because delay is a conglomeration of several
  important variables, it is a common and useful metric.
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  Bandwidth refers to the available traffic capacity of a link. All other things being equal, a 10-Mbps
  Ethernet link would be preferable to a 64-kbps leased line. Although bandwidth is a rating of the
  maximum attainable throughput on a link, routes through links with greater bandwidth do not
  necessarily provide better routes than routes through slower links. If, for example, a faster link is
  busier, the actual time required to send a packet to the destination could be greater.
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  Load refers to the degree to which a network resource, such as a router, is busy. Load can be
  calculated in a variety of ways, including CPU utilization and packets processed per second.
  Monitoring these parameters on a continual basis can be resource-intensive itself.
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  Communication cost is another important metric, especially because some companies may not care
  about performance as much as they care about operating expenditures. Even though line delay may
  be longer, they will send packets over their own lines rather than through the public lines that cost
  money for usage time.
  
  Network Protocols
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  Routed protocols are transported by routing protocols across an internetwork. In general, routed
  protocols in this context also are referred to as network protocols.
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• These network protocols perform a variety of functions required for communication between user
• applications in source and destination devices, and these functions can differ widely among protocol
• suites. Network protocols occur at the upper four layers of the OSI reference model: the transport layer,
• the session layer, the presentation layer, and the application layer.
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  Confusion about the terms routed protocol and routing protocol is common. Routed protocols are
  protocols that are routed over an internetwork. Examples of such protocols are the Internet Protocol
  (IP), DECnet, AppleTalk, Novell NetWare, OSI, Banyan VINES, and Xerox Network System (XNS).
  Routing protocols, on the other hand, are protocols that implement routing algorithms.
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• Put simply,routing protocols direct protocols through an internetwork. Examples of these protocols include
  Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol
  (Enhanced IGRP), Open Shortest Path First (OSPF), Exterior Gateway Protocol (EGP), Border
  Gateway Protocol (BGP), Intermediate System to Intermediate System (IS-IS), and Routing
  Information Protocol (RIP). Routed and routing protocols are discussed in detail later in this book.
  
   

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