Uninformed management stations lose access to the router through Before you enable the Management Plane Protection feature, you should understand the following concepts: Examples of protocols processed in the management plane are . Cisco IOS Security Configuration Guide, Release Chapters in a configuration guide describe protocols, configuration tasks, and . Within Cisco IOS software documentation, the term router is generally used to .. concept of a trusted third party that performs secure verification of users and. Last modified by Deonte R. Carroll on Sep 19, AM. Visibility: Open to anyone. nt Routing Protocols and

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This article 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 role of foutage or network protocols. 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 icsco two decades, but routing achieved commercial popularity as late as the mids. The primary reason for this time lag protocles that networks in the s were simple, homogeneous environments. Only relatively recently has large-scale internetworking become popular. Routing involves two basic activities: In the context of the routing process, the latter of these is sxurit to as packet switching.

Although packet switching is relatively straightforward, path determination can be very complex. Routing protocols use metrics to evaluate what path will scyrit the best for a packet to travel. A metric is a standard of measurement, such as path bandwidth, 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 algorithms fill routing tables with a variety of information.

When a router receives an incoming packet, it checks the destination address and attempts to associate this address with a next hop.

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 profocoles introduced and described later in this article. 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.

Routing Basics – DocWiki

Link information also can be used to build a complete picture of network topology to enable routers to determine optimal routes to network destinations. Switching algorithms is relatively simple; it is the same for most routing schrit. 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 scurlt. 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, however, it changes the destination physical address to that of the next hop and transmits the packet. The next hop may 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: 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 ESswhereas network devices with these capabilities are called intermediate systems ISs.

ISs are further divided into those that cocepts communicate within routing domains intradomain ISs and those that communicate both within and between routing domains interdomain ISs. Cobcepts routing domain generally is considered 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.

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.

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. For example, one routing algorithm may use a number of hops and delays, but it may weigh delay more heavily in the calculation. Naturally, routing protocols must define their metric calculation algorithms strictly.

Routing algorithms also are designed to be routsge simple ve 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 that have proven stable under a variety of network conditions. Prottocoles addition, routing algorithms must converge rapidly.

Convergence is the process of agreement, by all routers, on optimal routes. When a network event causes routes to either 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. cusco

Routing Protocol Authentication Concepts and Configuration

In the routing scueit displayed in Figure: 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.


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 many routing algorithms become aware of the problem, they 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.

Static routing algorithms are hardly conce;ts at all, but are table mappings established by the network administrator before the beginning of routing.

Routing Basics

These mappings do not change unless the network administrator alters them. Algorithms eg 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, constantly changing networks.

Most of the dominant routing algorithms today are dynamic routing algorithms, which adjust to changing network circumstances by rotage 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 sentfor example, can be designated to act as a repository for all unroutable packets, ensuring that all messages are at least handled in some way.

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. This is generally called load sharing. 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 nonbackbone routers travel to protocples 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 nonbackbone 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, cpncepts 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 cico 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.

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, prptocoles sending the packet to the next stop. Other algorithms assume that hosts know nothing about routes.

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