Internetwork Design Guide -- Introduction
Internetworking-the communication between two or more networks-encompasses every aspect of connecting computers together. Internetworks have grown to support vastly disparate end-system communication requirements. An internetwork requires many protocols and features to permit scalability and manageability without constant manual intervention. Large internetworks can consist of the following three distinct components:
- Campus networks, which consist of locally connected users in a building or group of buildings
- Wide-area networks (WANs), which connect campuses together
- Remote connections, which link branch offices and single users (mobile users and/or telecommuters) to a local campus or the Internet
|Internetworking Design Basics
Designing various internetworks
IP Routing Concepts
UDP Broadcast Flooding
Large-Scale H.323 Network Design for Service Providers
Subnetting an IP Address Space
IBM Serial Link Implementation Notes
References and Recommended Reading |}
Figure: Example of a typical enterprise internetwork provides an example of a typical enterprise internetwork.
Figure: Example of a typical enterprise internetwork
Designing an internetwork can be a challenging task. To design reliable, scalable internetworks, network designers must realize that each of the three major components of an internetwork have distinct design requirements. An internetwork that consists of only 50 meshed routing nodes can pose complex problems that lead to unpredictable results. Attempting to optimize internetworks that feature thousands of nodes can pose even more complex problems.
Despite improvements in equipment performance and media capabilities, internetwork design is becoming more difficult. The trend is toward increasingly complex environments involving multiple media, multiple protocols, and interconnection to networks outside any single organization's dominion of control. Carefully designing internetworks can reduce the hardships associated with growth as a networking environment evolves.
This article provides an overview of the technologies available today to design internetworks. Discussions are divided into the following general topics:
Designing Campus Networks
A campus is a building or group of buildings all connected into one enterprise network that consists of many local area networks (LANs). A campus is generally a portion of a company (or the whole company) constrained to a fixed geographic area, as shown in Figure: Example of a campus network.
Figure: Example of a campus network
The distinct characteristic of a campus environment is that the company that owns the campus network usually owns the physical wires deployed in the campus. The campus network topology is primarily LAN technology connecting all the end systems within the building. Campus networks generally use LAN technologies, such as Ethernet, Token Ring, Fiber Distributed Data Interface (FDDI), Fast Ethernet, Gigabit Ethernet, and Asynchronous Transfer Mode (ATM).
A large campus with groups of buildings can also use WAN technology to connect the buildings. Although the wiring and protocols of a campus might be based on WAN technology, they do not share the WAN constraint of the high cost of bandwidth. After the wire is installed, bandwidth is inexpensive because the company owns the wires and there is no recurring cost to a service provider. However, upgrading the physical wiring can be expensive.
Consequently, network designers generally deploy a campus design that is optimized for the fastest functional architecture that runs on existing physical wire. They might also upgrade wiring to meet the requirements of emerging applications. For example, higher-speed technologies, such as Fast Ethernet, Gigabit Ethernet, and ATM as a backbone architecture, and Layer 2 switching provide dedicated bandwidth to the desktop.
Trends in Campus Design
In the past, network designers had only a limited number of hardware options-routers or hubs-when purchasing a technology for their campus networks. Consequently, it was rare to make a hardware design mistake. Hubs were for wiring closets and routers were for the data center or main telecommunications operations.
Recently, local-area networking has been revolutionized by the exploding use of LAN switching at Layer 2 (the data link layer) to increase performance and to provide more bandwidth to meet new data networking applications. LAN switches provide this performance benefit by increasing bandwidth and throughput for workgroups and local servers. Network designers are deploying LAN switches out toward the network's edge in wiring closets. As Figure: Example of trends in campus design shows, these switches are usually installed to replace shared concentrator hubs and give higher bandwidth connections to the end user.
Figure: Example of trends in campus design
Table: Summary of LAN Technologies
Network designers are now designing campus networks by purchasing separate equipment types (for example, routers, Ethernet switches, and ATM switches) and then linking them together. Although individual purchase decisions might seem harmless, network designers must not forget that the entire network forms an internetwork.
It is possible to separate these technologies and build thoughtful designs using each new technology, but network designers must consider the overall integration of the network. If this overall integration is not considered, the result can be networks that have a much higher risk of network outages, downtime, and congestion than ever before.
WAN communication occurs between geographically separated areas. In enterprise internetworks, WANs connect campuses together. When a local end station wants to communicate with a remote end station (an end station located at a different site), information must be sent over one or more WAN links. Routers within enterprise internetworks represent the LAN/WAN junction points of an internetwork. These routers determine the most appropriate path through the internetwork for the required data streams.
WAN links are connected by switches, which are devices that relay information through the WAN and dictate the service provided by the WAN. WAN communication is often called a service because the network provider often charges users for the services provided by the WAN (called tariffs). WAN services are provided through the following three primary switching technologies:
Each switching technique has advantages and disadvantages. For example, circuit-switched networks offer users dedicated bandwidth that cannot be infringed upon by other users. In contrast, packet-switched networks have traditionally offered more flexibility and used network bandwidth more efficiently than circuit-switched networks. Cell switching, however, combines some aspects of circuit and packet switching to produce networks with low latency and high throughput. Cell switching is rapidly gaining in popularity. ATM is currently the most prominent cell-switched technology. For more information on switching technology for WANs and LANs, see Internetworking Design Basics.
Trends in WAN Design
Traditionally, WAN communication has been characterized by relatively low throughput, high delay, and high error rates. WAN connections are mostly characterized by the cost of renting media (wire) from a service provider to connect two or more campuses together. Because the WAN infrastructure is often rented from a service provider, WAN network designs must optimize the cost of bandwidth and bandwidth efficiency. For example, all technologies and features used to connect campuses over a WAN are developed to meet the following design requirements:
Recently, traditional shared-media networks are being overtaxed because of the following new network requirements:
Network designers are turning to WAN technology to support these new requirements. WAN connections generally handle mission-critical information, and are optimized for price/performance bandwidth. The routers connecting the campuses, for example, generally apply traffic optimization, multiple paths for redundancy, dial backup for disaster recovery, and QoS for critical applications.
Table: Summary of WAN Technologies summarizes the various WAN technologies that support such large-scale internetwork requirements.
Table: Summary of WAN Technologies
Utilizing Remote Connection Design
Remote connections link single users (mobile users and/or telecommuters) and branch offices to a local campus or the Internet. Typically, a remote site is a small site that has few users and therefore needs a smaller size WAN connection. The remote requirements of an internetwork, however, usually involve a large number of remote single users or sites, which causes the aggregate WAN charge to be exaggerated.
Because there are so many remote single users or sites, the aggregate WAN bandwidth cost is proportionally more important in remote connections than in WAN connections. Given that the three-year cost of a network is nonequipment expenses, the WAN media rental charge from a service provider is the largest cost component of a remote network. Unlike WAN connections, smaller sites or single users seldom need to connect 24 hours a day.
Consequently, network designers typically choose between dial-up and dedicated WAN options for remote connections. Remote connections generally run at speeds of 128 Kbps or lower. A network designer might also employ bridges in a remote site for their ease of implementation, simple topology, and low traffic requirements.
Trends in Remote Connections
Today, there is a large selection of remote WAN media that include the following:
Remote connections also optimize for the appropriate WAN option to provide cost-effective bandwidth, minimize dial-up tariff costs, and maximize effective service to users.
Trends in LAN/WAN Integration
Today, 90 percent of computing power resides on desktops, and that power is growing exponentially. Distributed applications are increasingly bandwidth hungry, and the emergence of the Internet is driving many LAN architectures to the limit. Voice communications have increased significantly with more reliance on centralized voice mail systems for verbal communications. The internetwork is the critical tool for information flow. Internetworks are being pressured to cost less, yet support the emerging applications and higher number of users with increased performance.
To date, local- and wide-area communications have remained logically separate. In the LAN, bandwidth is free and connectivity is limited only by hardware and implementation costs. The LAN has carried data only. In the WAN, bandwidth has been the overriding cost, and such delay-sensitive traffic as voice has remained separate from data. New applications and the economics of supporting them, however, are forcing these conventions to change.
The Internet is the first source of multimedia to the desktop, and immediately breaks the rules. Such Internet applications as voice and real-time video require better, more predictable LAN and WAN performance. These multimedia applications are fast becoming an essential part of the business productivity toolkit. As companies begin to consider implementing new intranet-based, bandwidth- intensive multimedia applications-such as video training, videoconferencing, and voice over IP-the impact of these applications on the existing networking infrastructure is a serious concern. If a company has relied on its corporate network for business-critical SNA traffic, for example, and wants to bring a new video training application on line, the network must be able to provide guaranteed quality of service (QoS) that delivers the multimedia traffic, but does not allow it to interfere with the business-critical traffic. ATM has emerged as one of the technologies for integrating LANs and WANs. The Quality of Service (QoS) features of ATM can support any traffic type in separate or mixed streams, delay sensitive traffic, and nondelay-sensitive traffic, as shown in Figure: ATM support of various traffic types.
ATM can also scale from low to high speeds. It has been adopted by all the industry's equipment vendors, from LAN to private branch exchange (PBX).
Figure: ATM support of various traffic types
Providing Integrated Solutions
The trend in internetworking is to provide network designers greater flexibility in solving multiple internetworking problems without creating multiple networks or writing off existing data communication investments. Routers might be relied upon to provide a reliable, secure network and act as a barrier against inadvertent broadcast storms in the local networks. Switches, which can be divided into two main categories-LAN switches and WAN switches-can be deployed at the workgroup, campus backbone, or WAN level. Remote sites might use low-end routers for connection to the WAN.
Underlying and integrating all Cisco products is the Cisco Internetworking Operating System (Cisco IOS) software. The Cisco IOS software enables disparate groups, diverse devices, and multiple protocols all to be integrated into a highly reliable and scalable network. Cisco IOS software also supports this internetwork with advanced security, quality of service, and traffic services.
Determining Your Internetworking Requirements
Designing an internetwork can be a challenging task. Your first step is to understand your internetworking requirements. The rest of this article is intended as a guide for helping you determine these requirements. After you have identified these requirements, refer to Internetworking Design Basics, for information on selecting internetwork capability and reliability options that meet these requirements.
Internetworking devices must reflect the goals, characteristics, and policies of the organizations in which they operate. Two primary goals drive internetworking design and implementation:
The Design Problem: Optimizing Availability and Cost
In general, the network design problem consists of the following three general elements:
The goal is to minimize cost based on these elements while delivering service that does not compromise established availability requirements. You face two primary concerns: availability and cost. These issues are essentially at odds. Any increase in availability must generally be reflected as an increase in cost. As a result, you must weigh the relative importance of resource availability and overall cost carefully.
As Figure: General network design process shows, designing your network is an iterative activity. The discussions that follow outline several areas that you should carefully consider when planning your internetworking implementation.
Figure: General network design process
Assessing User Requirements
You can assess user requirements in a number of ways. The more involved your users are in the process, the more likely that your evaluation will be accurate. In general, you can use the following methods to obtain this information:
Assessing Proprietary and Nonproprietary Solutions
Compatibility, conformance, and interoperability are related to the problem of balancing proprietary functionality and open internetworking flexibility. As a network designer, you might be forced to choose between implementing a multivendor environment and implementing a specific, proprietary capability. For example, the Interior Gateway Routing Protocol (IGRP) provides many useful capabilities, such as a number of features that are designed to enhance its stability. These include hold-downs, split horizons, and poison reverse updates.
The negative side is that IGRP is a proprietary routing protocol. In contrast, the integrated Intermediate System-to Intermediate System (IS-IS) protocol is an open internetworking alternative that also provides a fast converging routing environment; however, implementing an open routing protocol can potentially result in greater multiple-vendor configuration complexity.
The decisions that you make have far-ranging effects on your overall internetwork design. Assume that you decide to implement integrated IS-IS instead of IGRP. In doing this, you gain a measure of interoperability; however, you lose some functionality. For instance, you cannot load balance traffic over unequal parallel paths. Similarly, some modems provide a high level of proprietary diagnostic capabilities, but require that all modems throughout a network be of the same vendor type to fully exploit proprietary diagnostics.
Previous internetworking (and networking) investments and expectations for future requirements have considerable influence over your choice of implementations. You need to consider installed internetworking and networking equipment; applications running (or to be run) on the network; traffic patterns; physical location of sites, hosts, and users; rate of growth of the user community; and both physical and logical network layout.
The internetwork is a strategic element in your overall information system design. As such, the cost of your internetwork is much more than the sum of your equipment purchase orders. View it as a total cost-of-ownership issue. You must consider the entire life cycle of your internetworking environment. A brief list of costs associated with internetworks follows:
Estimating Traffic: Work Load Modeling
Empirical work-load modeling consists of instrumenting a working internetwork and monitoring traffic for a given number of users, applications, and network topology. Try to characterize activity throughout a normal work day in terms of the type of traffic passed, level of traffic, response time of hosts, time to execute file transfers, and so on. You can also observe utilization on existing network equipment over the test period.
If the tested internetwork's characteristics are close to the new internetwork, you can try extrapolating to the new internetwork's number of users, applications, and topology. This is a best-guess approach to traffic estimation given the unavailability of tools to characterize detailed traffic behavior.
In addition to passive monitoring of an existing network, you can measure activity and traffic generated by a known number of users attached to a representative test network and then extrapolate findings to your anticipated population.
One problem with modeling workloads on networks is that it is difficult to accurately pinpoint traffic load and network device performance as functions of the number of users, type of application, and geographical location. This is especially true without a real network in place. Consider the following factors that influence the dynamics of the network:
From a practical point of view, sensitivity testing involves breaking stable links and observing what happens. When working with a test network, this is relatively easy. Disturb the network by removing an active interface, and monitor how the change is handled by the internetwork: how traffic is rerouted, the speed of convergence, whether any connectivity is lost, and whether problems arise in handling specific types of traffic. You can also change the level of traffic on a network to determine the effects on the network when traffic levels approach media saturation. This empirical testing is a type of regression testing: A series of specific modifications (tests) are repeated on different versions of network configurations. By monitoring the effects on the design variations, you can characterize the relative resilience of the design.
After you have determined your network requirements, you must identify and then select the specific capability that fits your computing environment. For basic information on the different types of internetworking devices along with a description of a hierarchical approach to internetworking, refer to Internetworking Design Basics.