Routing basics

C H A P T E R
Routing Basics 5-1
5
Routing Basics
This chapter introduces the underlying concepts widely used in routing protocols. Topics
summarized here include routing protocol components and algorithms. In addition, the role of
routing protocols is briefly contrasted with the roles of routed or network protocols. Subsequent
chapters in Part 6, "Routing Protocols," of this book address specific routing protocols in more detail,
while the network protocols that use routing protocols are discussed in Part 5, "Network Protocols."
What is Routing?
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.
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.
Routing Components
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.
Path Determination
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.
Routing Components
5-2 Internetworking Technology Overview, June 1999
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 5-1 depicts a sample destination/next hop routing table.
Figure 5-1 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.
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.
Switching
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.
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. 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 5-2.
To reach network: Send to:
27
57
17
24
52
16
26
...
Node A
Node B
Node C
Node A
Node B
Node A
Node A
...
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Routing Basics 5-3
Routing Algorithms
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). 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. 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.
Figure 5-2 Numerous routers may come into play during the switching process.
Routing Algorithms
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.
Router 1
Router 2
Router 3
To: Destination host
Router 1
(Protocol address)
(Physical address)
Source host Packet
PC
To: Destination host
Router 2
(Protocol address)
(Physical address)
Packet
To: Destination host
Router 3
(Protocol address)
(Physical address)
Packet
To:
Destination host
Destination host
(Protocol address)
(Physical address)
Destination host
PC
Packet
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Routing Algorithms
5-4 Internetworking Technology Overview, June 1999
Design Goals
Routing algorithms often have one or more of the following design goals:
• Optimality
• Simplicity and low overhead
• Robustness and stability
• Rapid convergence
• Flexibility
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.
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.
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.
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.
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.
Figure 5-3 Slow convergence and routing loops can hinder progress.
Router 1 Router 2
Packet to
router X
t1
Routing table
Dest.:
X
Send to:
R2
Already updated
Routing table
Dest.:
X
Send to:
R1
Not yet updated
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Routing Basics 5-5
Algorithm Types
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.
Algorithm Types
Routing algorithms can be classified by type. Key differentiators include:
• Static versus dynamic
• Single-path versus multi-path
• Flat versus hierarchical
• Host-intelligent versus router-intelligent
• Intradomain versus interdomain
• Link state versus distance vector
Static Versus Dynamic
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.
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. 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.
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.
Single-Path Versus Multipath
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.
Flat Versus Hierarchical
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. Packets from non-backbone routers travel to the backbone routers,
Routing Algorithms
5-6 Internetworking Technology Overview, June 1999
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.
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. In very large
networks, additional hierarchical levels may exist, with routers at the highest hierarchical level
forming the routing backbone.
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). 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.
Host-Intelligent Versus Router-Intelligent
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.
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.
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.
Intradomain Versus Interdomain
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.
Link State Versus Distance Vector
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.
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 Basics 5-7
Routing Metrics
Routing Metrics
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:
Path Length
Reliability
Delay
Bandwidth
Load
Communication Cost
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.
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.
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.
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.
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.
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
5-8 Internetworking Technology Overview, June 1999
Network Protocols
Routed protocols are transported by routing protocols across an internetwork. In general, routed
protocols in this context also are referred to as network protocols. 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.
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. 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.